I'm talking about this one: https://www.cmmginc.com/shop/22lr-ar-conversion-kit-bravo-grey/

I just ordered one and I'm trying to think logically on why it says it wouldn't work in a piston upper. I don't plan on trying to retro fit it as I have a DI upper. I'm just trying to go through a thought process.

As far as I know these 22 conversions do not rely on gas returning to cycle the bolt. They are blow backs. They also do not rely upon the buffer/spring as the kit is a complete system.

The only thing I can guess is the piston might over extend thus not allowing the bolt/carrier to insert into the chamber.

I have a AA and HK MR556 piston system, and you can easily remove the piston. So I would think logically, a piston wouldn't be problematic to fit these kits.

What do you guys think or experienced with such a kit?

And no, I am not allowed to purchase any more uppers. So I can't get a dedicated 22.

You are correct on the piston. 

Remove the piston, but be aware 22 is dirty and will create a lot of fouling.  Had to clean the hell out of the AA GB to get the piston cup back in and rotating smoothly.

Don't know if my gun is the exception or not but this is what happened to me. I have a AA 16 inch mid length and purchased a 22 cal conversion kit from CMMG. no problem with the gun cycling. just turned the piston to 3rd setting suppressor. after 2 trips to the range i sold the kit and mags at a local gun show. accuracy out of a 16 inch barrel with a 1 in 7 twist was horrible. couldn't keep it on paper at 50 ft. this gun is a tack driver with 223 and 5.56 at 100 yards. even tried subsonic 22 cal ammo with no better results.

Thank you for sharing your experience. I am glad to hear it works on a piston upper. When you say you turned it to the 3rd setting, you mean the gas? Mine only has two but then again I'm not even sure why the gas returning is important at all?

It's not. The third position on the standard AA gas block is off, it blocks any gas from working on the piston.