Suspect thatit'd be a bit like MNPS - if the failure happens before entry, keep out. If it happens after entry, tell ATC and you will probably be allowed to continue - but don't expect to be able to change levels until top of descent! Because it's probably safer to keep you where you are than mess everyone else about.
Is it a case that you can't load the extra fuel you'd need to continue below RVSM airspace - or that you don't want to because it's uneconomic? That's a balance of risk that your opertaions planners must surely have addressed?