I installed retropie onto the CM3 and I'm running it through some performance tests. I have an old xbox controller I'm using. Bootup timees have been less than 30 seconds, which has been awesome (most zero based builds have a boot up time of about a minute. I think the eMMC module really helps disk speed). My next steps there are to get some emulators installed.
The coprocessor module is completely laid out and routed. I made some 1:1 print outs and have the parts coming from digikey. I might have to adjust the crystal and inductor footprint. I've been looking into OSH Stencils and might order a stencil from them. I have a discount code but I might wait for the whole board to get a stencil. It's only $6.
For reference, the idea behind the coprocessor board is to bring everything that would be on the GBA board normally. I can develop the code and debug any circuits on a breadboard. I'm excited to check the layout and send them off to OSH Park.
As a side note, in the future, I'd recommend ordering a CM3 dev kit from Adafruit. Their prices were much better.