Thanks charliem! I'm starting to think that USB is the root of the issue.
I fired up the apcupsd status panel and got the "could not connect to service" error; according to the system logs apcupsd was not started as the UPS could not be found. I unplugged the UPS, plugged it in again, checked dmesg and got the following:
ugen0.2: <unknown>at usbus0 (disconnected) uhub_reattach_port: could not allocate new device ugen0.2: <american power="" conversion="">at usbus0</american></unknown>I went into the apcupsd panel and hit "Save" on the configuration page then clicked over to the "Status" tab and lo and behold, the UPS service had been started and the status was printed out. This was a couple of hours ago; I've just checked the RRD graphs / vmstat and the wired memory has stopped increasing and leveled off, and no funny USB related stuff in dmesg either.
I've checked the UPS' USB port on my linux dev machine and all seems just fine, my guess is that there is an issue with the USB driver and the way it is initialising the USB hub which is hung off the controller on the motherboard. I've detached the UPS' USB for the time being, will restart it and see what happens. Incidentally I plan to upgrade the hardware to a more modern Haswell CPU / board in order to save some power so I'll do the same set of experiments once that's up and running in a couple of weeks.
Many thanks for your help! If things start to get wierd again I'll append this thread with an update as it does seem there may be an underlying issue with the kernel and this USB configuration.