Does anyone know of any authority for a constable (or sheriff, too, I suppose) to collect a 10% commission for selling real property under a writ of execution? The closest thing that I can find is section 34.047 of the CPRC, which says that the officer can retain his actual expenses for holding personal property prior to sale (definitely not a 10% fee for selling real property).

No. Decades ago that was the law, but the problem was that the fee that came with a commission sometimes would be highly out of proportion to the actual cost of providing the sheriff's services. This was especially true on the sale of high value realty. The statute was changed many years ago. There still exist some commission laws for certain other public servant tasks, but they are few and far between.