Is there anything left to be implemented here? As far as I know, the only feature the Osborne emu lacks is the ability to write to disks.
I just checked to make sure, and writing to .mfi disk images works fine. Use floptool that builds with MAME to convert images to .mfi format. For example:
make -j8 TOOLS=1 REGENIE=1 PTR64=1
./floptool64 convert auto mfi AMCALL.IMD amcall.mfi
./mame64 osborne1 -flop1 os1xutls.td0 -flop2 amcall.mfi
In the emulated Osborne1 I used SYSGEN to copy CP/M to the disk in drive B and setup to configure it for 104-column mode. Then to make sure the changes really persisted, I exited MAME and booted from the modified image:
./mame64 osborne1 -flop1 amcall.mfi
AMCALL booted in 104-column mode, while the disk image originally didn't even have CP/M present. Writing to .mfi disk images definitely works.