Jack,
The TO-300 is a fairly common transformer...
I may be mistaken but the Heath WM-4 may have used them... the amp in the auction looks very similar to the WM-4, iirc.
Of greater interest are the larger ACRO UL-60 (hope I have the number right) and the ST-120 which is a dual UL-60 on one chassis.
Ultra Linear is not "triode" exactly... there is a good write-up on Ultra Linear operation in Radiotron Designer's Handbook.
McIntosh is very different than all other common output designs, for a number of reasons. The biggest one is that it is unity gain. All others are not. (ok there are some exceptions...) The frequency response of the McIntosh is due in large measure to the way the output windings are configured - again Radiotron explains it nicely. But they do use feedback in the commercial version of the circuit...
Building today, I'd go true triode...
The idea of modulating the screens via a capacitive coupling is interesting... but a bit depends on how the screens actually work. If they draw current like a grid in AB2 or A2, then that won't work exactly the way one might expect... also the capacitive tap may cause some interactions between the screen supply and the plate supply... interesting though. Not sure I have seen this done anywhere...
_-_-bear
