Assuming your adapter complies with UL guidelines (many things carry fraudulent UL and CE markings) then when you draw an amp or so the AC/DC adapter should safely fail. Probably by melting a filament somewhere in the transformer, those things are too cheaply made to have fuses. The plastic case should stay intact and no high voltage should be exposed.
You will want to check the output voltage of the power supply to ensure that "regulated" means "close to the set voltage". AC/DC adapters are notorious for running well over their labeled voltage at low current.
If you have plenty of cooling capacity at 12v, you could switch your adapter down to 10 or 9 volts. The fans will turn a bit slower and be quieter.
You could put a 500mA fuse in the +12v line for the day when you move a piece of metal gear, mash the power lines to the fan with an edge, and short the wires. Maybe the fuse would blow before the transformer fails. Which goes first in a short circuit has many variables.