Because the ULA outputs are open collector with a pull up and pushed down by a keypress and corresponding address line you can do the same over the edge connector if you listen to the IN address ($FE) and can either use the same matrix or any circuit simulation a keypress. With that you can use internal and external keyboard at the same time or use several external keyboards and maybe develop a kind of game using keyboards.
There has been an official product from Memotech:
http://www.zx81stuff.org.uk/zx81/genera ... board.html
And here is an article of the german ZX team for adapting an AT keyboard to a ZX81 (german language only, beginning from page 11)
http://forum.tlienhard.com/magazin/1996/magazin_396.pdf
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