To be honest it isn't that hard to make a few Workbench-styled themes for some light-weight window manager, combine them with some scripts and create a deb, and then make a custom Debian install.
How competent are the linux forks of UAE? It could be as simple as a linux boot loader for AmigaOS in an emulator.
The problem though isn't the difficulty, it's the point. Many of the design decisions in Workbench made sense at the time, or were even cutting edge but would look very dated or even archaic against a modern desktop. Today things have moved on somewhat and I really question whether something that went even slightly beyond simply just producing a skin for LXDE or similar would be in any way worthwhile from the point of view of mainstream Linux users given the alternatives already in the market.
I'm not saying that there isn't the space for another player, I'm curious though as to whether or not there is enough space.
