Last I used USB keyboard and mouse was through subway and Poseidon on a A1200. It work great. You could not however enter the early boot up menu, as the drivers were not load at that point.
The drivers /are/ loaded at that point if you make them resident (or install them to flashrom) and given you have AmigaOS3.9 BoingBag 2 installed with updated bootmenu (which uses a system conform way to check the mouse buttons), you will be able to enter the boot menu and use the USB mouse (or keyboard) there.
As for the mouse emulation, everything has been said already
