The biggest issue is probably the thermal limit of the transistors and transformer. Most UPSes use very small heatsinks for the power transistors, the logic being the time they are supporting the load will usually be short. Example for an APC UPS:
https://surdu.me/assets/images/apc/bad-caps.jpg (not my image, found on the internet)
If you use them for a long time you may discover the FETs go pop shorting out your battery bank and blowing the fuse.
So at the very least I would suggest upgrading heatsink there and/or adding a fan. The transformer is a harder limit to work with, it is probably built a bit like a microwave transformer build quality wise, in so far as it only needs to run for 10 minutes at a time so cheap insulation and thin gauge wire will be used. Once the temperature gets too high and the insulation fails, game over.
Also typically the charge rate of the battery is not very good for consumer UPS, my one is for instance 10 watts when charging, which for a 7Ah battery takes roughly 8hrs to recharge. If you increase that 10x you are looking at multiple days to recover charge for another blackout.
A better option may be to buy a second hand server UPS and build your own battery array for it, or a true sinewave inverter; you could use your existing UPS to give you time to activate a manual or automatic changeover switch for such a capability.