One minor flaw - as you imply, you need to be paid at the higher rank for two years before you're entitled to the pension. You probably couldn't rely on the 12 months after you PVR - it's up to manning when you can be released, so you might find yourself retained for 11 months and 3 weeks and leaving on a Flt Lt's (PVR) pension.....
It would also depend what age you are and what job you go into when you get promoted. It's usually the job that carries the RoS, but be aware that Manning are wise to this sort of trick and will often make an offer of promotion contingent on waiving or extending an option.