The output side of the UPS is effectively a separately derived system, so I would expect the neutral to be grounded in the unit.
So a phase-earth short on the output is effectively a phase-neutral short as far as the UPS is concerned.
Many UPS systems (and small inverters in general) just won't supply enough current to trip anything but the lowest rated MCBs, so it would be a case of how to determine the touch potential at the point where the fault occurs.
As I see it, it will depend upon:
1. The potential divider effect of the phase and earth conductors.
2. What the output voltage of the UPS drops to under the fault condition.
#1 is easily calculated from the cable sizes, but point #2 will depend upon the UPS design.
Unless the output side has cables so long as to limit the current severely (in which case the voltage drop under normal use would almost certainly be far too great anyway), or the UPS is a really large type, then I don't think the average UPS is going to deliver anything like the 40 or 50V touch potential at the far end that would be considered a hazard.
There's nothing to stop you running the UPS output through an RCD if you wish.