have multiple input devices, via wsmux, and then make wsmux select the one you
want:

in XFree86Config:

Section "InputDevice"  
        Identifier  "Mouse0"
        Driver      "mouse"
        Option      "Protocol" "wsmouse"
        Option      "Device" "/dev/wsmouse"
        Option      "Buttons" "5"
        Option      "ZAxisMapping" "4 5"
#       Option      "Buttons" "2"
#       Option      "Emulate3Buttons"
EndSection

in kernel config:

# wscons pseudo-devices
pseudo-device   wsmux                   # mouse & keyboard multiplexor
wsmouse*        at pms? mux 0
# USB Mice      
ums*    at uhidev? reportid ?
wsmouse* at ums? mux 0

but if you only have serial, then use moused (8) to take the serial I/O and pass
to wsmouse.

in rc.conf:
wscons=YES