But, if the "wonder antenna" is small enough to be enclosed (mostly) in
something transparent to the RF but not to the thermal thats generated by
any I^2* R losses, wouldnt the temperture rise inside the enclosure give a
decent indication of efficiency?



No need for that really, too cumbersome. If you measure R you can calculate
losses due to dissipation in heat.

Again, real comparison of efficiency of shortened antenna is to compare against
the same pattern producing full (electrical) length antenna, everything is
included in what you would measure. Much simpler too. I can build quarter wave
vertical faster than styrofoam igloo. :-)

Basic rules are, best inductive loading is about 2/3 up the radiator, coil is
better than loading stub, top hat is the best. Anything else is jocker's
attempt at perpetuum mobile.

Yuri