A frame-mounted bashplate would be difficult with GS headers since they run almost under the frame tubes but with any other header it should be doable.
It's still a 3" drop from the frame to the sump. Even if you get it rigged you'll have trouble making it strong enough not to move without making it weigh a lot. I looked into to this once. I could've gotten a plate hanging off the frame but I'm pretty sure it'd get mashed into the sump in a hard impact. Let us know if you figure out a solution.
I've got big chunks of rubber matting behind my sump-mounted bashplate to soften the blow. It might be a more realistic solution for you. People have been bashing these bikes over all sortsa things for a long while with sump-mounted plates. I wanted to mount a plate to the frame too but the sumps seem pretty stout.