Just Use the ELT
These aircraft already have an ELT, and computerized fault detection. The ELT can already be manually switched on, or triggered by impact (searching the net I found two numbers, 5G and 2.3G) What could be done simply and cheaply is list conditions which are always an emergency and trigger the ELT automatically when those occur. For example, the aircraft:
* is inverted
* is descending at 10,000fpm
* has lost pressurization at high altitude
* has lost power
You get the idea. In one of those emergencies the pilots may never get to the last priority in Aviate - Navigate - Investigate - Communicate, but the ELT is just sitting around. Let it handle the communications.