I've been fooling around with waterslide decals, just have been waiting on wear experiments before I post results. I've mostly been doing top printing, as a secondary legend (Russian Cyrillic) but have done some side print too. So far they are holding up OK with a clear coat. Without a coating it scratch off too easy. Side printing shouldn't be much of an issue, since you don't touch them really. It's a good cheap and do it yourself mod with as much flexibility as your imagination and your inkjet can make.
I would not mind doing some as a test group for a few people to see how long they last under different use patterns and people. If you have some 'spare' key caps you would like printed PM me. You would pay for both ways shipping, and materials cost of $2 for as many legends can fit on an A5 sheet. You must have a design ready, I am not going to do that for you.