I'm very curious about what sort of work was involved in such a project. Presumably there was some sort of relatively low level compatibility layer involved, since Marklar was kept updated to run 10.2, 10.3 and 10.4, apparently with a small team (Quora post said the guy needed a project he could do alone).

There were at least two other successful ports of MacOS to x86 back in the pre-Steve days, both described in Jim Carlton's book "Apple." Presumably those ports were trickier since they were porting code originally developed specifically for 680x0 chips rather than originally developed on a portable Unix-like operating system. Still, they were able to get things up and running remarkably quickly. Performance and bug fixing and third party compatibility were another matter.