TI ship TCA8424 which pushes HID over I2C at 1mhz.
The chip is preprogrammed for standard laptop keyboards but supports up to 128 key/function inputs including multiple simultaneous key entry (great for all those function/ctl commands)
while it is built to support Win8, there is a lkddb entry for HID_I2C which may be possible to build into the Mer kernel adaption by default.
using this chip would only require physical hardware construction of the KB using the pin/wire connections as defined by the TCA8424.
i'm waiting on word back form lbt and Jolla and if they are good to go, I'll by e TCA8424 dev kit and start trialling.
I need to see if the HID_I2C driver can also push input directly into Malit.
stay tuned as more info comes to hand....

edit: if the kids give me break this arvo, I'll try a kernel build with HID_I2C and see if I can get it up on my Raspberry Pi with Mer. if this works, we'll take it form there...