Posted this to https://gitter.im/grin_community/crypto earlier but posting it here for a wider audience.
Nobody there pointed out any glaring errors why this is obviously flawed so please take a look and poke some holes in it.
I will post this to the mailing list in a day or so (assuming this is not completely broken).
Edit: Received some valuable feedback from @tromp and we discussed some alternative ways to improve the original proposal.
Latest version here -