I'm not sure what you mean by part 91 certificate. Anyway, this is from the MNPS manual if you do indeed have an MNPS LOA but not one for RVSM:
SPECIAL ARRANGEMENTS FOR NON-RVSM APPROVED AIRCRAFT TO:
- Climb/Descend Through RVSM Levels
MNPS approved aircraft that are not approved for RVSM operation will be permitted, subject to traffic, to climb/descend through RVSM levels in order to attain cruising levels above or below RVSM airspace. Flights should climb/descend continuously through the RVSM levels without stopping at any intermediate level and should “Report leaving” current level and “Report reaching” cleared level.
- Operate at RVSM Levels
ATC may provide an altitude reservation for an MNPS approved aircraft that is not approved for RVSM operation to fly at RVSM levels provided that the aircraft:
· is on a delivery flight; or
· was RVSM approved but has suffered an equipment failure and is being returned to its base for repair and/or re-approval; or
· is on a mercy or humanitarian flight.
Operators requiring such an altitude reservation should contact the initial Oceanic Area Control Centre (OAC), normally not more than 12 hours and not less than 4 hours prior to the intended departure time.
The altitude reservation approval should be clearly indicated in Item 18 of the ICAO flight plan. It must be noted that the provision of this service is intended exclusively for the purposes indicated above and is not a means for an operator or pilot to circumvent the RVSM approval process. The service will not be provided to aircraft that are not approved for MNPS operations.