You mean every 3 hectoseconds?
Uncommon combinations of multipliers and time units are made use of in a tongue-in-cheek manner in some communities (one VMS setting was measured in microfortnights), and in those pentaminutely would be guessed at and the unusual construction appreciated with smiles or groans.
Outside of those, it would not. Even five-minutely has issues, since minutely is rarely used (presumably because it can be confused with minutely meaning by a very small amount).
A great way of expressing this would be "repeating every five minutes". In some technical contexts, then "repeating with a five-minute period" would work well, but the sense of period that refers to a regularly repeating interval isn't as well known as other senses, so it wouldn't serve as well for all audiences.
Just say your edit on it being in a computer object model. pentaminute would be fine if your coding standards allow a modicum of humour in naming. Otherwise fiveMinuteRepeater or fiveMinutePeriod etc. is clear and to the point.
(Incidentally, hectosecond is an SI unit that complies fine with ISO 8000. Just don't actually ever use it for anything).