Anyone know if there are any AR15 pistol blue gun / replica / airsoft?

I need a non-firearm I can use to demo real accessories on an AR15 in pistol configuration.

Never seen one.  Fake guns don't have to abide by the NFA, so it doesn't seem like something to ever be made.

is there a decent airsoft / replica I could easily hack up? looks like some of the airsoft might be able to replace the buffer tube with a real pistol tube. and some might fit a real upper?

Use an 80% lower, you can install everything but the FCG.

^ This

Blue guns exist, we've used them in training. Exceptionally pricey, though. They had a version with an adjustable stock, so potentially you could remove it entirely.

Airsoft  replicas sometimes have dimensional issues, so be aware of that if you go that route.

Having an obvious non-firearm for a demonstrator has several advantages, esp in certain environments.

Also, there could be an issue with constructive possession if you have someone that likes tho push issues...