VoxelMap has for some time been able to display helmets players (and mobs) are wearing. With Forge, there can be many helmets besides the standard ones, with their own textures. VoxelMap has been able to display these as well for a while.

Sometime this year Forge added the possibility to define custom models for armor as well as custom textures. I just added the ability for VoxelMap to display these as well. It works fine for the one example of a custom model I can find so far (the Knightly helmet in Twilight Forest) but should work for most, unless they do something really weird.

This was a bit of a PITA and I'm not sure it was worth the effort, but I like it. Screenshot included!