You can set up a separate general-purpose system as the NUT server for
your "arbitrary clients", using dummy-ups
in "relay mode" as the driver.
This instance of dummy-ups
would in turn be the NUT client allowed to
interact with the appliance and that way with the UPS connected there.
The original question related to a NAS with NUT provided in its firmware OS, that only allowed one or few clients and not a whole rack’s worth of client IP addresses.