I recently bought a few of these
which is a light blackberry keyboard with touchpad, that connects via usb c to a host or can be used as a i2c device via multiple connectors.
I added a MDBT breakout module that fits pretty neatly into the free space inside the enclosure.
Used the Web IDE to connect wirelessly to the breakout board, which works seamlessly, if you remember or implement a few tricks to switch pairing once you start the BLE HID mode. The Espruino module connects to the great I2C interface of the original keyboard and does its own keyboard mapping etc.
There is enough IO left to actually connect the keyboard matrix directly to the MDBT breakout, but you would need to provide your own level converters etc., so this is a neat compromise for now.
I'll try to polish the pairing and security functions, but for general use, it's working quite well already.
Pretty happy with Espruino's incremental tinkering experience that lead to a viable solution for the problem.
I'll promise to re-do the ugly soldering next time I need to open the case...