If you presume as I do that Forseti and Fosite are the same God, the Codex Unia gives us the story of His appearance to the Frisians. The section tends to be translated as an exchange between "Charlemagne" and Redbad. However, the text itself is about "King Karl," and Karl Martel would fit in far better with the timeline.
I've been working with An Introduction to Old Frisian by Rolf H. Bremmer, Jr. (2009) and will be posting some admittedly loose translations here soon ("soon" being a relative word with this blog, granted).
In the meantime, here is a link for "The Tale of Charlemagne and Redbad" in Old Frisian.