Peter, I expect the output of the UPS is isolated from the source so any protection before the UPS probably won't work.
I cant see any reason why an RCD wouldn't work on the UPS output side, the RCD is only looking for a difference in current on either the active or neutral leg of the 240v supply. The only sure way to find out is to test it.
Edit. Just tested with my UPS and plug in RCD, Runs no problem, the test button trips the RCD with no ill effects to the UPS.
At least on my UPS, the second RCD neither stops the UPS nor trips the upstream switchboard RCD.
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