I have seen it done. It does work. But you ahve the correct thought of taking it out, testing-using the battery. The problems ive have seen are ,
- the charging system of the UPS wasnt ment for that kind load to recharge
- the ( even thought it will be outside of it) parts arent ment to be around the off gas of the wet cell battery
- the UPS Heat sink and regulaters were kinda designed around the shorter power outages (15min type)
Those are what ive ran into.