Hello Tom,
I've added $port['alert_array']['ifName'] = $port['ifName']; in ports.inc.php just to see if it all works as I wanted and -- yes, it does exactly what I need.
I of course agree this is not the right way of doing it, because it adds needless work every poll (which I'm doing every minute, instead of usual five) -- which brings me to next question, and I would be perfectly happy with checking it every hour.
Maybe Alerting facility could be extended with some low-load low-severity alert class, which gets checked every n poll cycles? Or some option similar to Alert Delay, like Check Interval.
Thank you for your reply :)
PS. Observium is great. I'm using it for 3 days and already can't figure out how I managed without it so far :)