Thanks a lot Ukki, I'll test it in a couple of minutes

Updated, it crashed a couple of times while saving/editing settings with this output:

"X Error of failed request: BadDrawable (invalid Pixmap or Window parameter)
Major opcode of failed request: 129 (MIT-SHM)
Minor opcode of failed request: 3 (X_ShmPutImage)
Resource id in failed request: 0x1c00010
Serial number of failed request: 579
Current serial number in output stream: 580
killall: dosboxbin: no process killed
ruby1.8[5434]: GLIB ERROR ** GLib-GObject - file gsignal.c: line 652 (emission_pop): should not be reached
aborting...
Aborted"

Now config works, but returning to dos prompt leaves you with no working virtual keyboard (it's there but it doesn't do anything, like its functionalities where limited to the program you just exited)
To have a working Dpad usescancodes==false appears to be the only way.