In the meantime I've indeed been working on 'retrokits' (finally found a use for the domain name www.retrokits.com which I had reserved for much too long) the Retrokits are (for now) rom expansion modules for the RX5. Here's a sample from the 808 version:
And here a picture of the real-life cartridges:
The carts on the photo; on the left there is the shapeways version without embossing and on the left there is a prototype from a Ultimaker 2 printer. The embossing on the ultimaker looks a bit lo-res but I'm confident they will look good on the SLS printers like Shapeways use, even in standard material.
The cartridges use a flash rom but a programmer to update/replace kits is a future plan which will depend on how many people are actually going to own a RK cart. For now I will compile some classic drum machines and probably contemporary sound carts, or carts on request for those interested.
but - thanks for the pickup on these 3D projects guys, I hope it will be of use to some people.