I'm no expert at UPS's, but you said something that made it sound like
you didn't realize something.  On a typical UPS (well, on ours, anyway),
there is NO switchover from AC to DC.  All the protected equipment is
ALWAYS running from the batteries (via an inverter), with the usual 
condition of also having them on charge.  If the power fails, big deal - 
the computers never see it (until the batteries start to droop, but 
there's something like 60 car-sized batteries in that cabinet, so it 
takes a while).

If you were gonna run the guts on straight DC instead of an inverter,
why not do it all the time?  Then there'd be no switchover to screw
things up, and no having to sense the failure fast.  Just keep the DC
on charge when the power is on, and it'll be there in zero time when
you "need" it.