After an entire morning of smoking horn drivers I have the following to report with this qualifying statement:
I discovered that I had discarded the diaphragms that had suffered from glue failure so I ended up substituting Eminence PSD2002-8DIA diaphragms. 8 ohms, 80 watts AES. Same impedence but 20% less power handling than my horn driver circuit so at least any fusing would be on the side of safety if there turns out to be such a thing. Test rig used included 1 channel of a Yorkville AP6040 power amplifier, Tone generator, Amprobe AC ammeter, various AGC type fuses from 1/2 to 4 amps in capacity, one idiot who likes breaking things in the shop so that they don't break on the job. The diaphragm was not installed in a driver so this did not take into account cooling of the air gap and magnet structure.
Test 1 was brute force worst case scenario: Amp gains to max, tone generator to max output @ 50hz , inserting fuses of greater and greater value until the diaphragm goes pop. The result was every fuse below 1.5 amps blew almost immediately, 1.5a fuse held its own and the diaphragm began so show signs of heat but then the fuse blew, the 2a fuse and the diaphragm blew simultaneously.
Test 2 same setup (with new diaphragm) but leaving the 2.0A fuse inline with the diaphragm and slowly increasing the output of the tone generator until the diaphragm gets too hot (heat one up and you can tell when it becomes unhappy) and various frequencies from 20 - 20khz. The result was that up to 1.3a the diaphragm was happily heated away with no signs of distress but by 1.5a there was definitely cooking going on. This was independent of frequency.
Since the power handling of my drivers is 100w and the test subject was 80w I'm going to try the 1.5a fuse in parallel with the cap as by bypass it so worst case if the fuse blows I lose my cap bypass but the cap will still compete the circuit. We'll see how this goes. Hopefully I never have to see if it works anyway.