Yes, it's mouse only, but in the comments section
I asked about keyboard support and the author of the SmallyMouse2 project replied that it's mostly a matter of interfacing it to the computer:
The advantage of quadrature mice is that, although the connectors are not standard, the output to the retro computer is, therefore it’s possible to make a universal adapter such as SmallyMouse2.
Keyboards are another issue entirely. Whilst the USB hardware of SmallyMouse2 is quite capable of interfacing to a USB keyboard; the real issue is the interface to the retro computer. They all use different keyboard layouts and different interfaces. Furthermore, to support a ‘scanned matrix’ style of keyboard (common in many machines) you need something like a crosspoint switch IC to emulate the matrix.
That’s the long answer, the short one is ‘not really practical’
Obviously the author doesn't have the time to create an interface solution for every vintage computer there is out there, but maybe someone here could look into that part?