There is a driver for serial mice on the Internet, so in theory you should be able to use the following setup:
One USB->PS/2 adapter, connected to a PS/2->Serial mouse adapter, connected to a DB9->DB25 serial port adapter.
I have no idea if this would work in reality, as the amiga's serial port isn't exactly complying to the standards...
It does work, i have used it with a trackball that didn´t work with my punchinello mouseadapter.
Obviously it doesn´t work with the early startupscreen or hardwarebanging games, but other than that it´s great.