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The route for a grounding wire in this application is somewhat complex. The
dormer described sticks out from the bottom of the "A" roofline about 12'
off of grade, so a ground wire from the mast would need to follow down the
outline of the dormer wall, back about 4' to the exterior wall (with metal
siding), then down vertically about 12 to a grounding rod.

As I understand it, non-vertical ground wires are less effective than
straight runs down. Curious to know if I could simply ground the mast to
the metal siding, and have a short grounding wire from the bottom of the
metal siding to the grounding rod? Use the siding as part of the ground?

You are correct about straight runs being better. I don't know if I
would use the metal siding as a conductor. Doing that may increase
the likelihood of stray current branches entering the house.