I've actually done this once before. It works. The rear derailer was used only as a chain tensioning device. The stop screws were set such that it was aligned with the teeth on the SS freewheel. This is a somewhat inelegant solution. You still need two derailers, or some kind of chain tensioner. And the chain shifts under higher tension of pedalling, if that is an issue with you. I think a three speed cluster at the rear makes a little more practical sense. It's easily done and customized with cassette cogs, only one derailer is needed (rear), and the chain shifts under consistently lower tension of the derailer.

Three speed hub is an alternative. Utilizing one may solve some problems, create others, depending on your needs.

The idea of a three speed cassette sounds interesting to me. Why three speed cassette? Has all the range I need, with lower maintenance of a single derailer, modest hardware demands, and ease of customability.