Could this work as a stretch goal?
Well, yes, but it would be an expensive one.
So if your store stocked RPis then you could add them as a pledge too? Not that you'd likely be able to compete with the existing suppliers on price, I suppose.
No, as it would still be a finished end customer product that is available somewhere else.
I'm undecided on this project personally, by the way. I already have a Retrode, and have ripped most of my carts and put them in storage by now. I use my Pandora for most of my emulation needs, and while the only video I can get out of my old unit is SD after I broke the LCD connector, that's enough for most systems, and soon I'll have a Pyra to cover me too. I can see how people who already use a RPi for emulation would be interested in this, but I suspect only the ones that don't already have a Retrode, or have one but haven't ripped all their carts to SD already. But maybe there's a bigger market for people wanting to use the original carts for nostalgic reasons - I suppose they might well already have all the bits needed to fit this case.
If you ripped them, you still have a nice small console with a controller port where you can load them directly off the SD Card.
You're not forced to use the cards.