before reading your full message, I was already composing my answer, built around something like "you can do both things, by...", but you successfully anticipated me! Anyway, I'm the one responsible for writing the "highlighting" feature, and I do recall that I had to disable the ability of selecting multiple terms in order to make it work. I then added the ability of editing existing terms specifically as a duct-tape solution in order to work around the limitations I (felt I) had to add.
I'm failing to recollect why allowing multiple terms right out of the box didn't work, though. I'd like to believe I had good reasons...
Right now, we are (still) extremely busy trying to build a version of EPPI-Reviewer that doesn't rely on Silverlight, so I'm afraid we are not too keen to write new features aimed at speeding up (relatively rare) activities. I will certainly keep in mind this suggestion when we'll reach the highlight facility for the next version, however, I can't at the moment provide an ETA, I'm afraid.
Your suggestion about combining "highlights" feature with the Machine Learning components is quite tempting, on the other hand. I will pass it on to our ML specialists, it might generate a few "new" ideas...
Thanks for getting in touch, I appreciate your kind suggestions and thoughtfulness.