I know this is an old thread but there is a lot of new info on the M&P 15-22 that isn't on our site but is on a lot of other sites. Rather than start a new thread this info fits with the OP's question

Battle Arms Development makes a specific ambi-selector for the 15-22.

The Norgon Ambi-Catch fits perfect for the 15-22.

KNS Precision Inc. now makes the Non-Rotate Trigger/Hammer Pins in both MOD 2 and original versions.

A few companies now make an upgraded charging handle, the one i got is the NDZ Performance, which has a lot of good rievews, the other is Plinker Tactical. Both of these companies make several parts for the 15-22.

Hope this info can help our members out, let me know if you have any questions.