Would this work? Using a computer SMPS as a DC-DC converter

I would not rely on the AC fuses in the PSU - you probably have some HVDC fuses kicking about with that much battery to hand anyway :-)

For a one-off you could just open it and remove the rectifiers. If you put a high enough voltage on the input then even an active PFC should not complain (it should make the average current proportional to the instantaneous input voltage while maintaining the average output voltage; i don't see a reason for failure here). It it does not have an active PFC then you should be completely safe.

Technically, any DC power supply powered from a PFC stage is a DC-DC converter, so your idea has some merit.

It mainly depends on the PFC controller IC. Simple ones (which look only at bulk DC for UVLO) should run. More complex ones which sample the input AC for waveform shaping may not work if 'correct' waveforms are absent. YMMV.

The input bridge AC rating shouldn't be far off from it's DC rating. You'd be passing all the power through two diodes, mind you, so they may get hotter than you'd expect (under AC, each pair of diodes in the bridge gets a break every half-cycle).

The warnings about DC fuse ratings are 100% correct. Batteries can be nasty.

If there's some exotic auxiliary supply (quasi resonant) that is expecting sinusoidal AC in, the whole thing may not start. Most ATX supplies aren't exotic, so I expect that the aux converter would be powered from the bulk DC voltage which should be fine.

Good luck!