This should do the trick and is efficient (only changes the timestamp format once per hour on the hour).

Code:
on *:START:updatetimestamp

alias updatetimestamp {
  .timestamp -f $chr(91) $+ $gmt(HH) $+ :nn:ss]
  .timerupdatetimestamp -o $calc(($asctime(H) + 1) % 24) $+ :00 1 0 updatetimestamp
}


I would imagine there would be some quirks to this method though (e.g., messing up on DST changes).

You could also consider a program like this that would make mIRC believe the time was different than your system clock.

By the way, it's not always safe to assume that the minutes are the same between your local timezone and UTC. There are several timezones around the world that are a half hour or even quarter hour offset from the "standard" timezones. Properly dealing with timezones in a computer program can actually be a surprisingly complicated matter.