what would be the easiest way to depressurized the aircraft from the outside?
The negative pressure valve is typically just a spring-loaded panel somewhere along or near the belly. The combination types (negative and over-press) are similar and both types are on all current commercial transport aircraft.
I've seen it done and the only force to be overcome aside from the calibrated spring would be the pressure diff x the area, which on a panel that small (particularly at field elevation) is quite manageable and the resultant outflow is not dangerous.
The valve can be held open, if needed. This operation limits any potential damage to that LRU.