I agree that under part 61 the 20 hour rule does not really apply, but can you imagine the scenario if a solo student was involved in an accident?
FAA would no doubt check student pilots training records and flying logbooks and ascertain that said student pilot had maybe received 1.0-2.0 hours with an authorised instructor. In addition to this the student has maybe ten hours of supernumerary flying experience with an FAA Private pilot. (The fact that this pilot is a JAA Instructor is irrrelevant to the FAA). As far as the feds are concerned said student would have been sent solo with 1-2 hours of instruction, and although there is no legal minimum, I'm sure it would be obvious to them that all the required content of part 61 could not possibly have been taught, practiced, and demonstrated in that time.
Now who would get the rap in this scenario? My guess is the CFI who signed the student pilot certificate and endorsed the flying logbook. The CFI does not 'have' to sign any endorsement, and in doing so pretty much absolves the flight schoool of any responsibility.