would this be safe?

Im running a vent in to my room from outside to help cool down my room and i was think about taking 2 120mm fans and a psu and pin mod the psu to run without being hooked computer, so would this be safe to run for long periods?

I wouldn't see why not....the PSU should be cool enough and the fans shouldn't put much of a load on it.

Only alternative I'd reccomend is buying a 12v transformer instead of using a PSU. A transformer would be silent and you could use the PSU elsewhere.

It should be safe, but like Bios said, a wall transformer would be a better solution.