thermal camera for radiating antenna?

Depends on the antenna, and the IR camera you use. You said 7/8 wave, but I think you meant 5/8 wave. I didn't check and of the VHF antennas I have, but I suspect you could see heat radiated from the coil assembly if you ran close to full rated power.

Looking at an HF antenna depends on the antenna. Some with a lot of metal mass around and inside the coil like a HiQ, take a much longer time to heat up than say a hamstick does. However, it is just as telltale where the along the antenna the heat radiates. Quite obviously, the loading coil is always first to show signs of heating, and then the matching coil if there is one.

If all you have is 100 watts of carrier, and the antenna in question is of decent quality, and the ambient isn't too cold or too warm, the usual time it takes to see a change can be as long as 90 seconds. In some case, like an ATAS or Lil Tarheel, it takes less than 30 seconds. Part of the delay is of course the camera.

I have a FLIR i5 (≈$1,700), and it does a decent job if you set things up correctly. The one I borrowed was a FlukeTi4R (≈$17,000). The big difference is their resolution. The Fluke will see changes as small as .1 degree C. Some nitrogen cooled ones, can do a magnitude better, but they cost upwards of $100,000!

After all is said and done, you have to download the images to a computer software program to sort of the real world temperature change, albeit the camera figures are darn close (at least in the Fluke)! The app however, allows you to adjust the pseudo color to better show the temperature change.

The bottom line is, it is nice to see where the losses are, but they're not definitive with respect to efficiency. You can assume that the quicker things heat up, the more loss there is, but you have to be careful as the total mass of the coil assembly has to be considered too. I plan on doing more testing, but I have to wait until Fall, and cooler weather.