Hi,
Real world - My view is that it's great to monitor what is going on, but useless if it tells you that it just blew up. The most important consideration, is to make sure that you always operate with the safe operating area. A glance back at the hundreds of postings on this topic. Clearly shows that the Lab Sub/Lab 12 driver combo, works well if it is within the SOA. People who have built the speaker and used it for years will confirm this. The next issue is making sure people don't overdrive it. A properly set up limiter with look ahead is the first answer. Next I would be looking at using an amp like the Powersoft K series or similar, which has user control of the output voltage.(see manual for description)
Homebrew funworld - First over-excursion. Whatever you can think of, somebody has already patented it or published a paper describing it. Laser sensing, magnetic positioning, optical encoding, extra voice coil with feedback, pressure sensing with a microphone(look at Meyer X10 Studio monitor, not what it's for but mounting idea), piezo sensor and more. Second, over-temp. Lawrence mentioned a cheap temp sensing kit and these are available from electronic hobbyist vendors. About $15 in the UK. You could put the probe in the air vent. But you need to be able to relate the pole temp to the voice coil temp. There is going to be a time lag between the two evening out. A number of people have tried putting IR sensors in holes drilled through the pole piece/magnet. That seems like a good idea. If you are going to be renting out the boxes, I would also suggest putting some temp indicating tape on the backplate of the driver. I am sure you use it in your installation business.
Iain.