Yep that sounds like the perfect thing.
But if its just the freezing problem. theres lots of ways to overcome that.
- save skinning, delete history so theres no connection to bones. Then freeze. Load skinning back on.
or - manually zero out root .r and .jo and offset the hip bone to match the original postion and orientation (can be done in several ways, im sure you know how)
or - write your own code that converts joint rotates to joint orients using the matrix information. Thats basically all that freezing does, but if you write it yourself you can make it work even on bound geos.
But to be honest. I thought that ART would work with yup as thats the standard maya world orientation.
Seems almost stupid to have to orient to zup just for rigging if all fbx import/exports convert themselves automatically.
The converison to zup in ue4 happens perfectly seamless on import anyway.