Between this and the SA-1 fixes for Gradius III and Super R-Type, we live in a golden age of improving SNES games :)
From what I can see, it looks like even though they can't extend the functionality directly, they do still interface with the system in some way. Perhaps they are more like a device connected to a serial port would be? So far less capable than the full extension cart, but still possible to communicate with through a standardized protocol? (The DS Cartridge slot has 8 data pins rather than the full address/data bus, but seems to have a protocol: http://problemkaputt.de/gbatek.htm#dscartridgeprotocol)
So, I need to correct myself, they nerfed it first with the DS, before then switching to being pure flash chips with the 3DS when they switched to XtraROM (which has potential lifespan issues: https://gbatemp.net/threads/nintendo-switch-3ds-cartridge-li...) - and that seems to not allow much custom functionality: https://www.3dbrew.org/wiki/Gamecards#Protocol
(Though if someone with more knowledge can correct me, please do so. I thought that DS/3DS/Switch are pure flash chips, but was wrong at least about the DS)
As ram, you could read the data on the cartridge, modify it, and thus fake bidirectional comms?
Amusingly the device that replaced my classic Palm was an Axim X3, which if I hadn't sold it to buy a server in 2005 would probably have already had your rePalm build installed on it.
1: perhaps more perfect analogy will be socketing ROM as its own chiplet if that ever made sense
Not that it was an official licensed product, but for what it's worth, there was a DS flashcart that bundled a significantly faster CPU than the DS's own - the SuperCard DSTWO [1]. The extra CPU (Ingenic Jz4740, MIPS) was primarily used for GBA emulation on DS/DSi systems without the need for a GBA slot passthrough flashcart, as was otherwise required - though there was a quite successful homebrew scene around it at the time as well, up to and including stuff like a (proof of concept) port of a PS1 emulator [2].
It was a surprisingly impressive device! The only downsides, as I recall, were the increased battery drain and the fact that the DS Slot-1 bus was only fast enough to allow streaming video output from the cartridge to the system's displays at ~45FPS, irrespective of the rate that the emulator was actually running at internally.
[1] https://wiki.gbatemp.net/wiki/SuperCard_DSTWO [2] https://gbatemp.net/threads/how-to-play-ps1-games-on-your-ds...
It's called the Didget blood glucose monitoring system, and I found it here: https://en.wikipedia.org/wiki/List_of_Nintendo_DS_accessorie...
https://en.m.wikipedia.org/wiki/Guitar_Hero:_On_Tour#Gamepla...
Edit: a better picture of it in action, https://en.m.wikipedia.org/wiki/File:Guitar-hero-on-tour-ds-...
You know 12 year old me never caught that.
At least Sagat kept his name, though "Tiger bullshit" would've been a great moniker, and no, I'm not salty about losing to his projectile once too many.
This leads to crazy modern enhancements like a Raytracing chip[1], or the MSU1 enhancement chip that is AFAIK not available as an actual physical chip, but only in software emulators. But it would be theoretically possible to manufacture, so you could have an actual physical SNES Cartridge of Road Blaster[2].
On the article itself, I noticed that his list has "Street Fighter Zero 2" as a USA ROM - that should be incorrect, since Street Fighter Zero is what Street Fighter Alpha was called in Japan. So Zero 2 should just be the Japanese version of Alpha 2. (Also, thanks for linking to the MVG video that debunks the myth that the delay before each round is caused by decompression)
[1] https://www.youtube.com/watch?v=2jee4tlakqo
[2] https://www.youtube.com/watch?v=BvIXUOr4yxU