ToME: the Tales of Maj'Eyal

By default custom tiles do not show equipment changes.

Is there a way to change that? (like, is there a bit I can flip in my save with a hex editor or something)

What precisely is different from the standard tile that prevents this (besides the shape. I'm mostly talking about the in-code stuff.)

I have a few years of programming experience, and I'm willing to check out the source to see how difficult this would be to add, but I wanted to ask first if anyone's already done the work. Obviously if you have a tile that's very much different from the default, slapping gear on it would look terrible. But I just want to be able to do small changes like make the default tile have the hair and skin color I want.

Race tiles have an associated attachment file for the equipment attach points, so every new tile will need an attachment file.
Individual race files have cosmetic options, have a look at the code for Ashes of Urh'Rok -races_cosmetic.lua for adding new options.