Using the term sandblasting generically.

Would an airbrush/glass etcher be suitable for prep work on an upper?

I have an older Colt M4 upper that is built on an A2 receiver. It happens to be grey while my Colt lower is the newer black.

It would probably take a bit to do it, but it should be okay. If you're glass beading, I'd suggest scrubbing the blasted areas with something like MEK & red Scotchbrite since glass beads will clean a surface, but leave it with a polished finish that some finishes can't stick well to. BTW, you may be better off to not take anodizing down to bare metal, but simply abrade the old surface a bit and scuff with scotchbrite as outlined above.

I looked at the small etchers because I have a small Makita compressor that will only do 3.6 cfm at 90 psi.  While looking, I found Paasche has a slightly bigger blaster that splits the difference between etchers and a full blaster Lac#3 that would work with my compressor.  I bought it, some 120 grit from Brownells, and a airline filter.  Don't know how well it works yet.  My plan was to blast an 870 express and then paint it with alumahyde ii.  Hasn't happened yet because the weather changed and its to cold to paint outdoors and I don't have a place to do it inside.  I hope to do it this spring.

I have an etcher for small parts like grips screws, the odd hammer, etc.  It would take you the better part of an hour or two to full blast aluminum oxide with such a wee unit.  They just don't have the sheer power to clean an upper efficiently.  I'd find someone with a bead blast cabinet and some glass to take the AlO₂  off of that upper, personally.

I bought my cabinet for $99 at Tractor Supply. No reason to waste your time or resources on anything else.

There are directions in the stickys for building you own "expedient" blasting box. I was given my cabinet (needing repairs), but there is no reason the blasting box above wouldn't work well. The airbrush blasters seem to be at their best for doing tiny jobs, doing a receiver could take a while, plus there's the cleanup to consider too.

Just for fun, I tried texturing my Sig P6 frame with an air etcher this week.

HOLY FAIL!!!!  I took two swipes and went straight downstairs to the bead blaster.