Good news: remapping the D-pad works
In an active session of Dosbox (i used the one I created to launch windows 3.1 with rubybox) ctrl+F1 will start the mapper, just remove the printscreen and the right alt bindings (they are mapped with the Up and Right directions of the Dpad) and then assign the four directions to the four arrows key.
Bad news: you can't save this, I don't know why (probably Dosbox points to a location that needs root privileges to be written), so this works only in an active session...
I just tried once though, so it could be possible that with some fine tuning of the dosbox config file one could be able to point to a different location where to save the mapping... or I was just lucky and I won't be able to make it work a second time