There is no such thing as a Part 91 OC, do you mean Part 125? If it is US registered, the LOA for MNPS, RVSM B-RNAV, P-RNAV must come from a US FSDO. FL280, otherwise. Why can't you get a RVSM LOA, if you have a MNPS LOA.

Just having dual FMS doesn't make you MNPS, there is a LOA for that, too. The only other option is Blue Spruce below 285, but I'm not sure if that airspace isn't MNPS, too

GF