Lame idea:
A third currency, FauxBux
FauxBux value = Robux for in game purchases, avatar market, etc.
FauxBux value = devex rate USD
Robux value = retail rate USD
Plugin pricing has 3 price settings: Robux, FauxBux, UDS
there, problem solved.
Bonus, you can see your devex-able account value by checking your FauxBux balance.
Before I get flamed, this is a joke 
(I think)