I haven't received any reports of the CF interfering with the compass, and it shouldn't. I have been flying with my compass attached directly to some experimental carbon fiber legs for a month now and have had no difficulty. I fly with a carbon fiber camera guard attached to them as well.
Thanks. So far so good, then. In general aviation, carbon fiber materials (fuselage, wings, etc.) as in the Lancair Legacy does present issues. So, thought I would ask around before mounting the typical plate between the gear legs.
For those that don't know, carbon fiber is a conductor.