Your idea is sound, but (depending on your design -- reads local degeneration), you might be better off with tracking temperature and resulting heat efficiency reduction.
Besides, it might be hard to electrically isolate heatsinks.
Here is an alternative. Mount each transistor on a copper block. Use electrical isolation between this larger block and heatsink. This will probably take you very close to your goal of lower heat loss through isolator. You pay a small price in input capacitance.
Another alternative is to go with something like Bergquist Silpad 2000 or functionally equivalent. People are getting quite religious about Aluminum Oxide washers. The best, however is BeO washers (electrically, dielectrically and conductive). BeO is banned in many places though and not available in all sizes (TO3 IS available, TO3P/247 and 220 was not last time I checked).
Petter