Dave - I'm absolutely not an expert on uniforms. I'm more or less constantly combing the web for uniforms, militaria, equipment, weapons, etc. I also use references like the Wooley books on German uniforms. I can only pass on info that I pick up from various sources, so what you're saying doesn't surprise me. Sure, I seem to answer a lot of WWI uniform questions, but I think I've always presented the answers with caveats. At least I hope I have!!!
Here's the original picture of the feldbluse.
I found this on a militaria site. Most of the time the militaria sites get the description of their materials right, but they make mistakes just like everybody else. This is the only one in that color that I've ever seen. So...take it with several large grains of salt please.
Looking thru the Wooley book section on Imperial Air Service uniforms, there are Sanke photos, posed studio shots, field shots with aircraft, you name it. I have to agree with you I do see at least a few uniforms that appear to be distinctive and look more like the feldbluse picture above. But what I think is more striking is the wide variation of uniform in the photos - cavalry, Hussaren, infantry, marineflieger abteilung, jager - you name it - the pilots and observers wore it. So, whereas you're apparently right and I was wrong about the existence of the German Imperial Air Service uniform, the impression given in this photographic record is that many more pilots and observers were wearing variations of other uniforms vs. those wearing the actual air service uniform. I don't think this is too surprising. In my experience (USAF) air crew, if allowed any variation or choice in uniform, will express their individuality. Pilots do not have trivial or small egos.
And that's a good thing.
All the best,
Dan