Currently, it is impossible to get proper cross-avatar support for games due to inconsistencies with the non-rig attachments found in respectively the R15 and R6 rig.
The below picture shows the difference - if you look at their hands, R6 is missing attachments that I am using for animation-purposes and accessories.
It is currently these two attachments that are missing:
- LeftGripAttachment
- RightGripAttachment