If it is just the forearms that can't grip, if you can get the nymph hanging upside down from the other four legs, that might even be enough I would think. They only hang from the back four legs when molting so the difficulty would be in getting to that upside down position and maybe also the flip to climb to safety after the molt is complete.
You could try creating some easy access routes to the mesh that aren't completely vertical, or simply put the nymph onto the lid yourself if it's showing signs of being ready to molt. And if you manage to catch it in the process, you could be on standby to help it complete the flip after getting out of the exuvia if needed.
Not sure if the above helps, just a few thoughts that came to mind. Best of luck to your nymph!