+1 on the WMB solution mentioned by gafftaper. I use that whole
system including the RF RAC and boy is it great. What I will often do as well when I am using Wireless Mic Belts is as a secondary measure to quickly
track packs and belts I will put a piece of white
gaff around the belt or shoulder harness and
mark the actor's name in black
sharpie. It helps
track the belts as well from beginning to end of the production so you don't lose any.
If you are looking to label the transmitter pack itself, a thought would be a thin strip of
gaff that goes all the way around the transmitter so the
gaff contacts itself. I usually wipe the pack surface with rubbing alcohol before applying a label to make sure there is no oil. Alternatively you may want to look at Brady labels or Ideal. They may have pre-made label packs that have a gooier glue. Along those lines, something with really sticky glue might be a thought... chances are very good it will end up a mess.
Other alternate methods - if you continue to find the transmitters are resistant to labeling and you are amenable to a permanent marking, a paint
sharpie in silver or white color could work but that seems like a fairly permanent step. And even more along those lines would be an
electric engraving tool with some paint to fill in the engrave.
Another thought - if the pack is really impossible to label I'd try to see if I could put a paper label around the
base of the antenna with some
clear heat shrink to seal the label in....