I almost laughed, but APC makes a 450va 1U rack mount UPS unit..... SC450RM1U
God only knows what this will run and for how long.
Turns out it will put out a massive 280watts for 4 minutes.
The only good news, is that at 25 or so pounds, it wont hurt anyone's back putting it in the rack. If I decide to sell it, I think the refurbished price would be that I pay you to take it.