My very first NES cartridge project
was effectively this idea.
I wasn't nearly as experienced with figuring out how to make designs low cost at the time. I was also suckered into the 'throwing high powered expensive hardware at it is fun'. The goal was to be low cost, but in the end with assembly costs it would have had to sell for closer to $100. That was the point where I realized cost could be kept much lower if USB hardware was off the board, and came up with my current product line up.
But my skills have much improved since then, and there's more and more hardware options available every year especially in terms of low cost USB. Most of my board designs over the past year have included foot print on the board to add USB functionality again, but dropping the footprint on the board is the easy part. I haven't been able to make time to complete the hard part of firmware and software to make it do anything. So still very much on my todo list.
Prior to getting USB back on a low cost board, I've been working on a new inlretro/kazzo design for most of 2017. A number of distractions in the later half of the year slowed my progress. But I'm pushing to release it by the end of this month. Having that complete takes care of large amount of the work for USB interface and host application. I should be able to port most of it over to a low cost USB capable mcu on the cartridge. That would effectively put the inlretro/kazzo hardware on the cart which is what I gather you're wanting. I'm no where close to final design, but I'm confident I can get it close to the $50-60 range in a polished retail product with enclosure and all. One pie in the sky idea I had for it was to integrate it into the action53 contributor's cartridge. The hope being it would help motivate people to submit an entry and effectively get rewarded with 'free' development hardware. IDK when I'll have things well enough together to make that happen though, I certainly can't promise it for the vol4 cart. Perhaps it could make it to vol5 though..
Anyway, I'm just trying to say I'm working on something along these lines, but can't estimate when considering all the projects I'm working on. But by all means I would support your development efforts if you were looking to take a project like this on for yourself. All the software and firmware for my updated inlretro/kazzo release will be open source, so if you utilize a similar mcu (STM32) it might help make short work for you.
EDIT: I just realized you're talking about SNES, my reply above was assuming you were talking NES.. To be honest I did really think there was a desire for much of SNES development hardware beyond what's already available. But SNES is continually gaining more homebrew interest. For some reason I've really been itching to get into SNES development myself, but I think that's primarily due to me wanting to better understand the hardware. So know that I realize you're talking SNES, I'll say that it's entirely possible for me to apply my NES plans of a USB dev cart to SNES. It's probably safe to say I'll tackle NES first though.
Byuu has made some requests for me to make a 21FX
SNES bootloader device available for purchase as a dev tool. I may target a cartridge pass through version instead of exp port device though due to complications of aquiring/manufacturing a SNES EXP port female connector. If I can make that project a reality, it would effectively make the pass through device USB capable. With that you could simply plug one of my standard SNES flash boards into the pass through device and gain the features you're after.