I needed to do a "headless" build of an FPGA where the software wasn't ready yet, but the RF guys needed the FPGA operate as if software was there and running the show. I did a sort of PSM as you call it that was like a really basic processor. Peek, Poke, Test, Jump, Inc, Dec. It was very helpful for debug, and the RF guys were able to make progress weeks before software was ready to go. Plus the software builds were very focussed on flight, and not doing special configurations to help others figure out corner case issues.
For a hobby project, depending on the goals of why one is doing the project, I think it's a fine approach. Gets results fast which helps with motivation and momentum to learn more difficult things. It's also a natural bridge for people coming from a software background.