Texas

The process is difficult requiring extensive research, submitting a nomination form to the Texas Historical Commission, review by committee, and then final approval by the National Park Service (I don't know how they are involved).

Since the home is in the historical part of San Marcos (I lived there for a brief time in 1988), I know that there are city regulations that prohibit the placing of signage without the city's approval so the couple is probably out-of-luck with your idea. The chances are even slimmer if there are any descendants of the Zimmerman family still live in the city.