I see from the wiki page on the DX (
https://wiki.mobileread.com/wiki/Kindle_DX) that the USB chipset supports OTG --- is this supported by the kernel? And is it present on all models? (And if so, is the socket powered?) As I'm going to have to get one off ebay there's no guarantee what I'll actually end up with.
A Kobo looks good, particularly as it looks like it's easy to build and flash my own kernel. It's a bit of a shame that the socket's not powered, though, because that will require an external battery to run the keyboard off.