I investigated this once, and it seemed like the answer was no. I am still skeptical, so you can always ask the BATFE:Would a hinged buttplate in the open position (like the M1A stocks) count towards the length? If someone made one for the AR platform?
https://www.ar15.com/archive/topic.html?b=6&f=50&t=436221
It would be a very nice workaround if you could get it approved.