No I'm not making one but I thought of how to make one. Just one of those thoughts that come to you y'know. And after seeing the Donnie made from a GSA60 it's inspiring.
If you can get a JS100 TR and find a GRX-720 neck. As I'm sure the GRX neck is straight like a JS
not angled with a scarf joint like a Jem RG.
If you widen the trem cavity, pickup cavities and neck pocket then it should be ok. The only thing that would be slightly off is the Treble side neck pocket as the neck would overhang the body slightly.
Although it would probably be easier to just get a custom made since your having to buy new trem and pickups anyway. but for someone with a lot of spare bits it would be a cool project.