Hi Andy, and welcome to the nuthouse.
I, too, am hoping to do a little overseas marshalling next year, so would be interested in any details you can find. This is something I'm hoping to expand on my website.
I'm not sure how useful this is, but the Catalunya circuit has a marshalling application form. I don't remember if this is just for the GP or for other meetings. The form is in Spanish, but even I can understand most of it.
The URL is
http://www.circuitcat.com/ingles/index.asp
Best of luck,
Paul.