Guernicus Terram magic - written up anywhere?

Hello folks,
Is the Guernicus Terram magic quasi-cult detailed anywhere?
It's mentioned in HoH p68 and I'm not sure if it is written in any other source books. If not I guess I'll write up my own version.

There's a bit on it in The Sundered Eagle (page 67) - Hedyosmos is a major centre for a branch of it.

Jarkman did some work on these guys.

Thank you you all. Just the style of material I'm hunting for!