Hi Kelly,
Nope, there's nothing wrong with that. In fact, if you look at how Xorg packages X11/XCB stuff, they like to have a separate package for headers, libraries, and binaries. This results in separate packages for `name-proto`, `libname`, and `name`. So you certainly wouldn't be the first to add a header-only package. Spack is for managing anything necessary for your installation, including header files.
Adam