Thanks everyone for your input and advice.
We called the aircon people and they said no need, they only put on around 5% of inverters nowadays and thats only when a customer requests. No mention of vibration effects. Seems this just made their own guidelines up here and rolled with it.
We told them everyone has them in the area - all surrounding rooms - then came the fear story - they expand and shrink in hot weather so after a year the back will be higher than the front on south facing aircons so they will become unstable, so you'll need to replace after 1 year - you still need?
We reminded them that they did the aircon next door and that has, and same brand inverter, they replied they stop putting them in the last year due to the above. Forever saving face, amazing persistence. I am okay with servicing it every year as part of cleaning and pressure check if they need replacing, maybe add a few hundred baht per service if that gives me a quiet vibration minimal aircon, it sounds as weak as I dont put oil in the bike because after six months it will be dirty and need replacing anyway.
Then okay okay, next time we're in the area we'll install, but in the meantime they are sending 2 to me so I can fix this week as next time in Thai is nondeterministic - if I cant do it ill have to wait till they are in the area or pay someone else. Best I can do with these people. Some progress anyway.
Apparently the ones they are sending are one per bracket - so two per aircon - rather than 1 per bolt (or 4 per aircon)