Well that is the same machine that I have. I guess by reading your post you don't have the manual. I will be home on the 11th of May so if you may want a scan or two of a page let me know.
My machine looks basically like yours here's a link to picture of it (for some reason my computer won't let me post it now)
http://img.photobucket.com/albums/v234/adammil1/ca0830f8.jpg. However the one thing my toolmaster is missing is that fork you describe. I haven't seen what you have done, but I've taken apart my mill. On mine when I withdraw the spindle all the way up, the top of the drawbar on mine protrudes about 1/2inch from the hole at the top and is enough to get my wrench around. However I rarely change tools as I have an ER40 collet holder.
None the less here's my advice, I don't know what you have or haven't tried yet you seem to be hinting at playing around with the drawbar but I'm not sure. Just to make sure we're talking about the same thing, I took the liberty of editing the picture to show you where the drawbar is.
You referenced it being the drawbar yet sounded a bit unsure. It works just like the one on a Bridgeport except for Cincinnati did you a favor by instaling a button to lock the drawbar, as you rotate the spindle with a wrench at the bottom of the spindle just above the toolholder. If you look at your picture "http://i54.photobucket.com/albums/g102/cyclone_65/cincinnati/P3290042.jpg"
Here's the pocedure's that I'd take if it were me who was fixing the machine.
1. First make sure the machine is in neutral, by hand you should be able to rotate the spindle with great ease. (If you didn't have the spindle in neutral that could be part of your problem)
2. If your spindle was in neutral, now go to the top of the machine and remove the 5 setscrews holding on the top cover so you can get a good look inside, this way you will also have more of the drawbar exposed to hold with a wrench or something if the forks won't quite engage properly.
3. Place a wrench around the bottom of the spindle on those two flats, and with the drawbar locked turn the spindle to loosen the threads.
One thought with out thinking too hard on the geometries, could it be possible that when you turn the spindle you are turning the wrong way? If the threads must be turned lefthanded to loosen (I believe these are standard right hand), then shouldn't you be turning the spindle right so that the tool holder loosens away from the threads? I could be wrong here, I can't think off the top of my head what way the threads should turn.
If all this doesn't work, maybe the damn thing is just stuck. With out looking in the manual I can't say for sure, but all this is is a drawbar, and it should work like all other drawbars! Maybe try jerking on the wrench but if that fails, perhapse you'll need to just use some real force. Put a pipe around the wrench handle to get good leverage, and as long as you have that drawbar locked pull. Worst case senario I would think would be that the threads are rusted together eventually if you can place enough force on the rod either the rust should give or rod snap.
One other thing to try, if you take the fork's cover off, so you get to the drawbar as I described above, with a good wrench placed around the toolholder, loosen the ram and slide it backwards. You should have it so that when you turn the spindle the wrench handle is jammed firm against the collumn of the milling machine. Then if you can get a wrench around the drawbar try turning the draw bar as opposed to the spindle. I don't know how good of shape the back is, but don't be shy, the one or two times I've done such things on my mill for repairs, I'll stand right on the table to get maximum leverage.
One other funny thing about the machine to note is under the back motor is a grease zert, the darn thing when the head is at 90deg is real hard to get a grease gun to. So when ever you tilt the head definately always add extra grease I've found. Other then that you have a really fine man's machine. An older machinist friend of mine says his dream shop would have a Toolmaster as it's mill and a Monarch 10EE as his lathe!
Sorry if my advice may sound a bit childish. If you did all that I'm stumped. I'm 99.999% sure with out looking at the manual that that is a plain and simple drawbar! So it should loosen just like any other with the exception that I believe you turn the spindle oppisite of the direction you'd normally turn the drawbar.
Adam