Maybe this is an idea. When the maid was hoovering around the legs of my deskchair (shoo, shoo, girl, can't you see bossy is busy?!), I couldn't help noticing a tiny window on top of her vacuum cleaner. Whenever one of my papers would block her nozzle an orange coloured piece of plastic appeared in that window. Activated by underpressure, it's in fact an indicator that tells her when to replace the dustbag inside by a new one. Wouldn't it be nice if we had a similar indicator that informs us whether an airfilter element is still good or has reached the end of it's lifetime. I believe the type of plenum that's on my CB 500 offers a possibility for just that. Have a look at the picture of that plenum, if you like. At the bottom there's a small tube attached ment to drain any fluids from the plenum. What if I - temporary - extend that tube and attach a simple vacuummeter at it's end. I could go for a testride and check at various speeds and RPM's if there's any significant change in underpressure in the plenum. I could compare the values of a new element with those of the old ones I still have and proof they're still 'open' enough. Now I myself will not ride shortly (it's still too cold here) but if anybody wants to perform this experiment, I will be very interested in testresults. Just a tube and a simple manometer. If you have a set of manometers (to sync carbs), you could use one of those. Or am I talking crap and will differences be to small to notice?
After defining the treshold value one could change the vacuummeter for a whistle that starts whistling whenever an airfilter is done (kind of pivot tube)
(No officer, I wasn't whistling at you)