This is certainly an interesting theory, but I'm not sure I would give it a great deal of credence without a lot of backup. I suppose a Frisian connection is not out of the question, considering their relative importance on the North Sea coast around the time Beowulf was being set down. OTOH, some of the setting in Beowulf would seem to be in Scandanavia. Perhaps this has more to do with the Frisians maintaining such a strong cultural independence throughout the medieval era (as discussed by Johannes and Thiu elsewhere).

As for possible Nibelungen connections, I'm even more sceptical. The Volsung/Nibelung story has almost as much mythic gravity in Germany as the Arthurian cycles do in the English speaking world. There is a strong tendency for "unusual" ideas and theories to get attracted to the story. As a case in point, there have been a couple of works trying to tie the whole thing to the area around the town of Soest and the river Lippe.

Still, nothing should be dismissed out of hand and I look forward to what you come up with and what Thiu has to say.