Since you are going to eating food cooked in that microwave, I've got to admit, I'd be careful about what I spray the inside with. I'm sure it'd be fine as described, but you never know what's going to leach out during an active cooking cycle where the microwave radiation is bombarding and spot heating.
Maybe I'm just being paranoid, but saving a $ or $$ won't be worth it if something happens...
The other thing is that since different polymers, and synthetics react differently to both uneven heating, and acids/bases + heating (afterall that's how we discovered that BHP leaches out of #1 PTFE plastic bottles... acidic solution (apple juice) + heat...)...
I'd just make sure you are considering all the potential risks... then go for it!