Dear Bethe,
I really think that your market is growing smaller by the minute. If you can get your film processed reliably en route, why wouldn't you? And if you can't, why would you risk it?
Besides, cumulative X-ray damage isn't a sudden 'either/or'. It's a question of what's acceptable -- and as I have yet to see indisputable X-ray damage in carry-on film, I suspect that it's almost always acceptable.
You'd need a different gauge for each film (not just speed, because of crystal variations) and you'd need several per trip if you wanted to check at each stage.
And, this is only much use with more than (say) four X-rays, and how many people run that risk on enough trips to make it worth while?
As I say, I don't wish to be a wet blanket, but I can't see any significant demand even if the testers were free. Certainly I wouldn't bother after 53 years flying, 40 since I took up photography.
Cheers,
Roger