Well, it wasn’t quite as bad as I thought it would be. When the headblock tie first broke, I had visions that it could mean tearing up the turnout and re-laying it. However, once cooler heads had taken over, a good solution presented itself.
I cut a length of .005 brass sheet, and bent it in an Omega shape to pass over the throw bar and under the switch rod:
This I then glued with gel CA onto the roadbed, thereby holding the throwbar down, and indeed arguably securing it in position better than the original design.
Finally, I after reducing its thickness with a Dremel sanding drum, I glued the broken headblock tie back in place. Apart from the brassy roadbed, which will cover up easily with paint and ballast, nobody need ever know that I had this problem.
Except you, of course. You know, because you’ve been reading this.