Paul

If I understand correctly, what you are doing is inserting a tommy bar into the hole that the chuck key goes into, and by holding the bar, preventing the chuck from spinning in the morse taper in the tailstock.

I would not have a concern about doing that on my lathe - the throw is 6", so a tommy bar inserted into the jacobs chuck key hole is long enough that it would wedge against the bed ways if the chuck started spinning.  However, if the lathe throw were any greater, I would worry about the safety of that approach.

While I prefer keyed chucks, a keyless chuck is actually safer for use on the lathe- if you inadvertently leave the key in the chuck, and the chuck starts to spin, it is possible for the key to be flung out of the chuck, possibly hitting you in the face.   That's the same reason that I was taught many years ago that you should attach the chuck key for an electric drill to the power cord - as a reminder to make sure that the key is removed from the chuck before you plug in the drill and start using it, and also so you can use the chuck key if the drill is plugged in.

The same thing could happen if you use a tommy bar to hold the chuck to keep it from spinning - if the lathe has a large enough throw that the tommy bar doesn't wedge against the bedways, it is possible that if you let go of the tommy bar, the spinning chuck could launch it into the air hitting you or a bystander.

The tendency for a jacobs chuck to break free of the morse taper and start spinning is a serious problem with larger forstner bits.  The largest bit in my collection is 2";  I think it is sometimes better to use a scraper or gouge to cut larger holes rather than try to drill them. 

My standard approach to drilling is to grip the jacobs chuck with my left hand. The issue there is that if the chuck starts to spin, the knurled surface can go a job on your left palm - it may not break the skin, but it does hurt - DAMHIKT;  to keep that from happening, I wear a kevlar carver's glove (with the little rubber dots to enhance the grip) on my left hand when gripping the chuck.   

I discovered..guess how... that If  I want the hole to eventually be a certain size, I need to drill out by many steps. I got a whole set of Forstner bits that increase by 1/4".

Took a while to find the really big ones, but they are out there.

The idea is to take small bites and not pushing in too quickly at the first point.  I push in a bit to set a "ledge".

Then as I go  ahead, the ledge guides me so it cuts without drifting or spinning.

Also, backing out often to clear the bit helps.

Paul
I like your idea.

It has happened to me many times the first several years of turning and getting the wood gone.  At first I needed the practice on using the tools and how to cut it all out and which tools were the best to do that job.  Then I started to using the forstner bits and had your exact same problem which is to much surface to cut using big bits.
Then like Ralph said I would start out with a 1" bit and go to 1.5 then 2" and up from there to the 3" which is the biggest I have now.
The bits are coming in contact and cutting a lot less wood so it does not spin.

Since I have not cored yet I would love to know where you are getting the larger forstner bits at.  Let me know please and hopefully it is not China.

I haven't tried them yet (because of the price) but I'd like to try the Fisch brand "Wave Cutter" forstner bits.   They're made in Austria.   Has anyone here used them and can give a review?

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Stumpy Nubs (James Hamilton) did a review of Fisch forstener bits a while back and gave them a very good grade. I have been wanting them ever since. 

Ed
I respect your opinion but don't agree with it.  There's a lot of us, myself included, who have had injuries in the past that affect us today.  If everyone was in perfect health, I likely would agree or at least consider seriously your comments.  If you don't have full use of your hands, you likely will either stop or find ways around it.  I have and I'm happy with the results.  It may not agree with everyone's methodologies but it's up to each person to decide what works and what doesn't.  It's no different than if someone need crutches or a walker to get around, it's just a fact of life.
Happens I ran across a strap cramp I'd forgotten about.  I may try that next time instead of the tool to hold the drill chuck steady, we'll see how that works.
Regardless as long as a person knows their limitations and work within it, let them enjoy their accomplishments.  I did furniture restoration and repair for a lot of years and I'm proud of what I did.  It may have been done in ways others may not have done it, but it got the job done.

Arlin

If I understand correctly, your suggestion is to make a draw bar can be used to secure the jacobs chuck to the tailstock ram.  There are two problems with that suggestions.

First not all lathes have a hollow ram in the tailstock that permits use of a drawbar.  The ram in my midi lathe is a solid piece of metal, so a drawbar cannot be used.  I can use a drawbar in the headstock, but not in the tailstock.

Second, a drawbar secures the male morse taper to the female morse taper, but in the case of jacobs chucks, that doesn't mean that the chuck itself is secured.   In many jacobs chucks, the taper is actually a double-ended arbor with an MT at one end, and a different taper (typically,  JT-33) at the other end.  While you can secure the arbor to the female taper, it is still possible for the JT-33 to 'break free' allowing the chuck body itself to rotate (or worse yet, come off altogether - DAMHIKT). 

I don't know what the original reason was for having this arrangement, but when I upgraded from a small lathe with MT-1 tapers to a midi with MT-2 tapers, it made it possible for me to upgrade my jacobs chuck by replacing only the arbor, thereby saving a few bucks (which I then promptly spent on other stuff).