On one of our science flights a fews years back someone on the team in the back got all excited about a reading and wanted us to turn around for another "sniff". As it turns out, the airplane flew through some ash and did some damage to the engines. A detector? Damage will already be done, so this will only prompt maint to do an inspection (if the damage is not severe enough to cause an engine shutdown, or worse), and maybe save the guy behind you (see the following story).
I was flying from PAFA-KONT when an Alaskan volcano was demonstrating it's thing. The ash cloud was clearly visible on a severe clear day with no other clouds visible. I gave a thorough PIREP, a KLM 744 checked in, center asked them if they heard my PIREP, they said yes...and then promptly flew into the ash cloud. After getting some power back on the heavy glider, they limped to PANC where there was reportedly $80 million in damages on the relatively new 744.
I'm not sure how practical this might be in a real world application given that after the fact is after the fact.