Just got off the phone with an Operating System expert consultant I hired through UpWork.
For Mr. Boehm to really be usable, the OS for the Pi Zero will require a static file system and boot in under 8 seconds, with an ideal boot time of less than 2 seconds... in order to power the Mr. Boehm environment off the Atari joystick ports.
Short version of our conversation, is that the consultant thinks it could be done based on Raspberry Pi blog posts he's read such as:
Basically, in addition to a Mr. Boehm pHat, we're looking at developing a fast-boot Mr Boehm OS for the RPi Zero.
The consultant gave a high estimate of about 40hrs worth of work. If I had to, I could probably cover 1/4 of the cost, but more than that, I'd likely need to doing some sort of Kickstarter project to come up with the rest. Whether there would be enough of an interest for such a thing is unknown to me at the moment.
Off topic, David sent me an updated schematic of the Mr. Boehm pHat board last night, and he's making progress. Working out all the connections for the actual PCB have been difficult, but he thinks he's about 80% done. Hoping within the next week or so, we'll have an update to show everyone.