Chris,

   Your analysis of the problem seems to reflect the problem I've seen with my 320 over the 800 hours I've been flying.  Its only occurred perhaps 3 or 4 times but it certainly gets your attention.  My procedure to get the gear down is to pull the gear circuit breakers (HI and LOW), quickly open and close the dump valve,  and push the gear circuit breakers back in.  Every time, the pump came to life and cycled the gear to the down and locked position.  Obviously, some type of back pressure locking is occuring on the system.  It happens very seldom (maybe once every 200 hours) and it doesn't seem to be related to any particular condition.

Don