An idea whose time has gone! Just what I was looking for.
The USB typewriter is a non-destructive mod for a mechanical typewriter that converts it into a keyboard that can be used with any Mac or PC – it’s shown here being used with an iPad, which is the only productive use I’ve heard of for one of those.
The conversion does require some sanding of the typewriter’s crossbar which makes contact with the keys’ arms.
You can purchase complete USB typewriters (current offerings are $400 or $450), a prefab kit ($150), or a bag of parts ($75) to solder together yourself. For $200 you can ship your own antique typewriter to the maker and have it converted.
I would love to pair an old portable typewriter with a mini-ITX or Via computer and a little LCD screen for the world’s weirdest and most wonderful laptop. Throw in a retro telephone handset for a cell phone, and look! A mobile vintage office!
Installation instructions and more information at the USB Typewriter site.