It might be more practical to simply copy the old technique for documenting buried water well locations, which was to take measurements to the two nearest corners of the house, making a triangle that will allow anyone with a tape measure to pinpoint a valve box.
Another practical no-high-tech-required idea is to use larger valve boxes. While I realize it is Texas custom to put each valve into its own tiny round box, as opposed to systems with large rectangular boxes holding half a dozen valves, it might serve you well to use
a certain large round box, originally made by Ametek, to contain one or two zone valves. While there are many similar boxes, the Ametek design allowed the placement of a 3-inch extension piece on the existing box, for when turf buildup finally threatens to completely bury old boxes.