I am envisioning that the breakage occurred in the female part of the fitting, where the 5/8 inch male spigot goes into. I would not use wire, but rather a semi-stretchy NYLON-based thread, like the aforementioned fishing rod wrapping thread, such at that made by the German thread-making giant Gudebrod. OR, using 4- to 6-pound nylon monofilament fishing line. Semi-stretchy (from 2% to 24%) wrapping materials like fishing rod wrapping thread and fine-diameter fishing line are very easy to wrap around obstacles, and exert considerable pressure when applied more than 10 wraps or so...plus they are almost invisible...they also can be extended beyond the broken area, where they can add some structural support. The idea is to apply pressure that will pull the broken points together. Used in combination with a top quality glue or cement, "wrapped" breaks on many substances can be patched with rod-wrapping thread, or fishing line, and the repairs last for years. Wire, on the other hand, does not exert much "inward" force, compared with the way semi-stretchy filaments like fishing line do. Wire tends to loosen its grip over time. THread and fishing line can be sprayed or painted with fishing rod building epoxy coats, and the repairs become in fact stronger than the un-broken material.
This all seems to be a moot point, however, since the Paul C. Buff people can fit a new, replacement mounting foot onto the light...a part that for example, Speedotron makes out of UN-BREAKABLE metal pipe...and which might add 50 cents to the cost of the light...