SO I'm pretty sure my idea will work:
add a keyboard to my router
http://h-wrt.com/en/doc/kb
then forward the keystrokes using netcat like in my edit link above.
I didn't have a good version on my router - I'll try some more when I get time. I just thought someone might have done this before and am just looking for pointers. USBIP might be a solution also, but might require more work. If I can just run a KUAL script to start netcat and receive keystrokes, seems simple on the Kindle end.
EDIT: OK, more info. Openwrt has many versions. Because I an old Belkin router, I need to use a 2.4 kernel version if I want wireless (I do), as the 2.6 version wireless is broken. HID support is only in the 2.6 version (of course!) so it looks like one or the other. A more current router would be a simple solution - probably not going to happen soon.
For now I abandon this project, but it does look quite doable.
As an aside, I've also seen USB infrared receivers that could be attached to a router, which could then be forwarded to a Kindle to do ... ANYTHING!