I'm considering a general purpose 'dimmer'. (more on-off ATM, but the functionality could be added later).
The power control device would be a 400V FET (that I happen to have a bag of that a pick and place machine ate half of the drain wire) inside a bridge rectifier on the live side.
A FET, due to the inherent current limit, which is nice as it's coming from a UPS that might otherwise trip on a fault when on battery.
Of course, isolated by a couple of optoisolators, with power coming from a small cap to neutral. Turned on by one opto, and the other indicates an overcurrent has happened, and the FET is off.
Assuming I can turn the FET on fast enough and off fast enough that it never gets hot, even if shorted, and I have an overvoltage device across the FET to catch spikes, and adequate snubbing across the bridge to stop the diodes being destroyed, is there anything I'm missing?