It is built-in, sort of. When you run apcaccess, it returns the latest status page from the device. The status page is updated about once a minute (about every 62s on mine), as timestamped by the "DATE:" line. The "MAXLINEV:" and "MINLINEV:" lines give the range of line voltage since the last status page (or possibly since the last one that was asked for... the documentation isn't very clear).

Edit: In fact some of the other documentation says it's the range since apcupsd was started. But if so, all you probably need to do is run apcaccess at *:59 and restart apcupsd on the hour...

Peter