1) Can someone provide an overview of the crossfeed system - in particular the detachable joint that fed LOX to to the shuttle from the tank?
The buzzwords to use for googling this topic are "ET Umbilical".
The system as implemented on Shuttle was complicated but caused major problems only once in the program (see below).
There were two umbilical areas towards the rear of the shuttle belly. Five fluid pathways connected through these two umbilicals: 17" liquid oxygen and liquid hydrogen lines, 3" gaseous oxygen and hydrogen repressurization lines, and a smaller hydrogen recirculation line. There were also numerous electrical and data lines. Large electrically operated doors (latched open at launch by electrically operated latches) covered the umbilical openings and also the aft structural connections between the ET and orbiter.
The outside of the doors was covered with shuttle TPS tiles.
At launch, obviously the doors were open and all umbilicals mated (the actual mating surface was called the ET umbilical plate). Each fluid connection had a shutoff valve on both sides of the interface.
After the SSMEs shut down, the ET separation sequence began. First the valves on each side of the fluid connections closed. Three explosive bolts on each umbilical plate fired to free the interface. Then triple redundant hydraulic actuators pulled the orbiter side umbilical plates into the Orbiter below the outer mold line. The system then checked to ensure that all valves were closed. If not, a waiting period began to let the systems blow down through the open valves and a warning was issued to the crew. Finally pyrotechnics blew the structural interface apart and the Orbiter fired down-firing jets to fly away from the ET. Then the pilot flipped switches to release the centerline latches on the doors, close the doors, and latch them closed.
The concerns with the system as designed were pretty obvious from the description above. If the fluid disconnect valves on the ET side did not close, the residual prop from the ET would vent out. If the ET was mechanically separated while this happened, it would tumble, and could hit the orbiter. Hence the waiting period. Failure of the umbilicals to retract into the orbiter could prevent the doors from closing, which would result in an imperfect heat shield for entry. This same concern existed for electrical or mechanical failures in the door closing mechanism. There was a documented EVA procedure for the crew to go out and crank the doors closed manually. This was straightforward except for accessing the area which was far aft and underneath the orbiter belly. There was also a concern that the disconnect valves could slam closed during operation of the SSMEs resulting in destruction of the vehicle. During the safety upgrades carried out after the Challenger accident, pneumatically operated latches were added to hold the valves open.
2) Would it be efficient if reused for 'asparagus-staged boosters'?
I cannot answer this, but my opinion is that the shuttle system design was unique to its application.
3) Or was it plagued with enough problems - or even just too expensive or inefficient for such use?
Despite the risks involved, the only actual major issue I am aware of with the system occurred during the STS-35 launch preparation, when the umbilicals experienced massive hydrogen leaks that delayed the flight significantly and resulted in the stack being rolled back off the pad into the assembly building.
1988 Shuttle News Reference Manual, p. 142 fwd and p. 92 fwd