i thought i would update this since no one had any helpful suggestions in case someone else searches the forum in future for a similar problem:
electrical tape didn't come close to pretending it would help, so that was a bust. at a friend's recommendation i tried "heat shrink tubing", which are rubbery tubes you can use for wiring and such. this may have worked better if i had appropriate equipment (i used a lighter instead of a heat gun and i had to cut the tubing because it was too small to go around the earpiece and i definitely wasn't going to cut the headphone wire), but as i tried it, it went awful. i say awful because it didn't just not work, it came close enough to working that i kept at it and when i'd finally given up the headphones looked in worse shape than when i started. if the problem was in the middle of the wire i think this would have worked great, but being at the joint there was very little for the tube to "grab onto" at one end and this actually using the headphones would subject this area to a lot of movement requiring it to be pliable.
then isaw 'performix liquid tape' at the hardware store. this is like a paint on electrical tape. the brush that came in the bottle was way too large so i repurposed a small make up brush which i cleaned with acetone between coats. i covered the exposed area and went a couple cm further down the wire, and then reinforced it all the way up the white back of the ear piece and the results were great. earphones still sound great and more importantly, the coating seems secure, has a little bit of flexibility to it, and has shown no signs of separating or cracking after several weeks of use. since the joint between the wire/earpiece is what separated, i went ahead and reinforced this area on the other side with the liquid tape as well (much less obviously), something i think anyone might want to consider as a preventative measure!
