After strolling a little bit too much around google I finally found something easy dirty and useful to connect serial input to Xinput.

As it is hard to compile (dependencies, errors, a lot) I found a precompiled debian/ubuntu package here http://dl.stackfoundry.org/softwedge/

All yu have to do is to type this in a terminal inside your X session. All input will be instantly available on the focused window.

softwedge -c /dev/ttyS0

Obviously substitute ttyS0 with your favourite serial emulated port such as rfcomm0 (bluetooth) or ttyUSB0 (usb serial dongle).