A year and a half ago I picked up some of the aluminum poles that match those fiberglass ones (the mil surplus stuff). The original idea was to use the fiberglass piece within the concrete as a base, and use the rest as the actual flag pole vertical. The boss wasn't too thrilled with the esthetics, and I really didn't want to strip the old Army green paint off of the aluminum...
Still have them, plus a could of rings that can be inserted within the system for guy wires. Not as high as the tree has the "L", but I'm thinking that in a pinch, it could be pressed into service as a temporary vertical.
I may just take the whole thing with me to Field Day and see what we can do. Or not.