Hey sorry to necromance @Bananacb, but did you ever figure this out?
Oftenticious did do this for the R15 in Apoc 2, but did not go into detail about it. DataBrain did this successfully with the even more eccentric R6. From this post.
I believe you need to utilize the actual texture of characters, which is the composite texture, in short when considering the uvs of custom characters. DataBrain also mentions the offset and how he dealt with it.