It’s pretty common to need this kind of bootstrap in the embedded world, and a similar technique is known there as the “three-instruction Forth”[1]. There is no actual Forth on the target, just a serial monitor with peek, poke, and jump, small enough that you can hand-assemble it in a pinch, but given a good dynamic interactive environment on the host you can build yourself a pretty comfy exploratory setup on top of that in a matter of days (hours, I imagine, for an embedded programmer who actually knows what they’re doing).
[1] https://pygmy.utoh.org/3ins4th.html