I would definitely buy a kit as long as it was reliable and uses mags that hold at least 30 rounds.
Even if it costs $300, it'd still be worth it because .22LR and 5.7x28 give the same results at the static ranges. They punch small .22" holes on paper all the same. You may get a bit more range with the 5.7x28, but most static ranges are limited to 100 yd, well within reason for .22LR
.22LR is about 1-2 cents per round, compared to 30c-40c per round for 5.7x28. I understand a lot of people are reluctant to use anything but 5.7x28 in their PS90 and come up with the wildest excuses, but I fail to see any negative impact whatsoever for use at the static range or for plinking.
I cannot afford to shoot at trash using 30c+ exotic Belgium made ammo on a daily basis, but .22LR does make a whole lot of sense and should allow for max enjoyment of the PS90.
A reliable .22LR kit would also increase the popularity of the PS90 because many people who did not want to pay the high prices for the 5.7x28 ammo can now afford to shoot .22LR
The 5.7x28 ammo snobs need to remember that not everyone is willing to pay 30c+ per cartridge just to shoot for fun. Price is one reason why .22LR is the most proliferated cartridge around the world.
I know if there was a .22LR kit available, I'd pick one up and blast several thousand rounds out of my PS90 immediately.