To answer my own question....
I found a claim like this somewhere "out there"....
A non-lockup TC absorbs ~2-7% of total engine power due to the "slippage" that exists between the impeller and turbine.
Assuming that you had a real good TC, on the low-loss end of that claim, 2% loss in efficiency seems excellent, and I would guess better than any existing sled transmission. Hmm... there may be a new experiment being planned out in the back of my brain.
Also note: A TC using hoses with a valve block would make for a very clean reverse.
This approach would also eliminate all torque related chassis stress, need for aligning the engine, allow significant weight reduction due to less motor mount, no chaincase, etc.
And of course, throw in a spring, piston, and shock absorber somewhere on the high-pressure side of things and all motor roughness would be absorbed.