All in all, I think this is a fairly successful way to make a multicart. It's user-friendly, flexible and cheap and I'm pretty happy with it. Also, people are still making and selling games for the Vectrex, in the form of real physical cartridges, usually with a bit of OTP EPROM on them. By using a processor-based approach like I did, for only a few bucks more, games can have a fair bit more of storage and processor power. I would very much like to see games that make use of that in the wild.
I would actually go and try to make a kick-arse arm-powered 3d game myself, but unfortunately I'm moving away form my current home pretty soon and I don't really have a way to safely take my Vectrex with me. That is also why I won't release the PCB artwork of my cart: as is, it's pretty bodgy, and I have no way to test an improved version. I do offer all of the source code for my multicart and the two demos in the same sentiment (and under the GPLv3, as usual): it's not really polished, but should anyone wanting to expand on the concept with a starting point. As always, if you manage to do something interesting with the sources, I'd love to hear about it.