I was hoping to add "Recreated ZX Spectrum Keyboard" support to this core but I think the USB keyboard support is in the MiST firmware that would need changing not the core itself. This keyboard, in "Game layer" mode replicates the ZX keyboard exactly, like Symbol Shift, Caps Shift but in Qwerty mode acts just like any normal keyboard but with special key combinations to get things like function keys, control etc. so Symbol/Caps Shift don't work in the core.
It sends a down character and up character for each key in "Game layer" as documented in http://sinclair.recreatedzxspectrum.com/downloads/recreated_sinclair_zx_spectrum_developer_guide_placeholder_ZX%20Keyboard%20Technical%20Document_a15_v1_1a.pdf
. I can't see a way to do this in the core as the firmware supplies the PS2 keycodes, not keyup/keydown codes, so would need access at a lower level. The latest Fuse emulator supports this keyboard.