I don't know much about the sampling capabilities for apcupsd. But "continuous" at some point must be translated to a discrete sampling interval.
Granted. OK. Here's the problem:
When the power browns out, it's usually out for less than a second. This would imply a sampling rate of around 10 times a second to be in with a fighting chance of catching it, which is going to rapidly fill up my disk with logfile.
When the power browns out, apcupsd is notified immediately. so it would make sense to grab MINLINEV at this point. Except that it's not reset between events.
Oh, and we've just changed the UPS so that it's not so sensitive, which means it won't trigger. Hmmm.
I guess what I need is something that samples (say) 10 times a second, but only logs the value when the line voltage is outside a defined envelope (or when it transitions between in-spec and out-of-spec, maybe).
I'll look at cooking something up later...