This version finally fixes the custom model exporter.
Basic workflow is to line up the models, transfer weights, Either use the make custom model exportable button to change what model it changes in the Prim, or just combine your custom model onto the og model and delete the og model in edit mode, this way it alreay has the names and data for exporting, then split by UV button (So that the seams show up correct ingame) And then opionally pad your custom model to match the og model vertices count, you dont have to but it helps with some of the backend stuff that gets run on exporting a custom prim. Then just export as normal with custom model topology ON.