In 1991, Loadbooks USA published "The Complete Reloading Manual for the Thompson/Center Contender." It was a compilation of data from the major bullet manufacturers and publishers of reloading manuals. For the .45 Colt, data from Hornady, Nosler, Sierra, Speer, Lyman, RCBS and Hodgdon are included, all of which except Nosler include heavier than normal .45 Colt loads. None of them make any distinction between octagon and bull barrels. I believe the octagon barrels were still being manufactured in .45 Colt at that time, but I'm not sure. None of this is directly from T/C, of course, but it is a bit hard to imagine all these bullet and powder companies generating this data without any interaction with T/C regarding what is appropriate. On the other hand, I do seem to recall reading something from T/C years ago about straight walled pistol cases. Their recommendation, as I recall, was just to be happy with the increased velocity resulting from longer barrels and the lack of a cylinder/barrel gap. Of course, that was when fast powders were more in vogue and before the Contender, which was conceived as a light weight field pistol, got grips and bull barrels which let it handle recoil. And, unfortunately, I can't find that article or letter, which ever it was.
I suspect the octagon barrels will handle published .45 Colt loads, but for me, the word brutal, which someone used to describe the recoil, is an understatement.
RonF