sounds like it will have issues when you approach large-as-f*ckness with your games.
and then comes the limitation of getting these made. It's for hardcore tinkering, IMO
Not so much for hardcore tinkering, it's purely for the development of new, larger games which will function in both regions from the cardslot. It's best to think of it like the MBCx mappers on the GameBoy, rather than a flashcard for playing commercial games.
There's no issue with running larger games from the card, unless you're trying to play commercial games on it. In which case you could only go up to 256KB (since the swappable bank defaults to mirroring the first 256KB on reset). If you're writing a new game, you can use all 8MB of the extended address space.
plus it doesnt fit in a jewel case
It is completely feasible to fit all of the components on a 4MB Plus card into something the size of a System Card 3.0, but of course that's only for actual manufacturing and not development.
What happened to this?
Not very much, actually.
I made a few custom 4MB ROM + 512KB SRAM MCGenjin boards for developers from some of my old testboards, and may do a proper layout of this configuration later in the year. Tomaitheous noted that something like that could be used as a new CD-System card, and I do like that idea.
Other than that I put together an additional notes document here:
http://www.penguinet.net/TailChao/Hardware/PCEngineKit/MCGenjin_SP.txt, which contains some information about a suggested header format and some mapper "gotchas."
Emulator support was hinted at, and I will try to find the time to look into Mednafen's source and add it. But right now I am occupied by work and a new Lynx project, and have only really used my own 4MB Plus cards for some library tests and backing up the BRAM on my TG16-CD.
But of course, if any new things are done with the mapper they will be uploaded here:
http://www.penguinet.net/TailChao/Hardware/PCEngineKit/index.php and I'll post the updates in this topic.