I've run into all sorts of of uses for headstones in my cemetery travels, but that was a first for me. I did hear of a homeowner near Laurel Hill Cemetery in Philadelphia who has headstones laid into the inside wall of his basement. Even as recent as the 1940s, people just discarded old broken tombstones. There seemed to be little desire for historic preservation.
A friend of mine has his study floor tiled with small white marble headstones from an abandoned nuns’ graveyard. All of uniform size, all beginning the name with “SRM” (for “Sister Mary ____”). Apparently, a nunnery (with a graveyard out back) was abandoned at some point near his home in the 1970s. As the property fell to ruin, one enterprising individual decided to make use of the headstones. My friend is probably the third owner since the installation.
Johnson Cemetery "Park" in Camden New Jersey, are actually laid-down headstones from African-American Civil War soldiers who are buried under the park. At Philadelphia's Mount Moriah Cemetery, you'd be surprised to see that the 'gravel' used to make up the roadways is actually bits of marble monuments and headstones. I've seen the same at the Metairie Cemetery in New Orleans.
When Philadelphia destroyed Monument Cemetery (near Temple University) in 1956, headstones and grave markers from about 28,000 graves were hauled to the Delaware River and dumped in, to be used as "rip rap" (rubble from building and paving demolition commonly used to protect shorelines from water or ice erosion). Ah, but the city planners had further use for these monuments and tombstones (which were whole, not broken up) - they eventually became part of the foundation for the Betsy Ross Bridge (construction was completed in 1976)! You can still walk to the shoreline and still see them sticking out of the water (read about the rip rap atrocity here.)
"Lost Civil War Graves of Johnson Cemetery Park" by Ed Snyder
History of Rev. John Wilson from Maryland Historical Society
The History of Plumbing in America