The n800 used to use something similar to a transparent Xcursor theme. Looking on the n900 it looks like /usr/share/icons/default/cursors/ has lots of symlinks to a similar transparent image - it may be possible to point one of these to an actual arrow icon to make a cursor show up everywhere.

It wouldn't have any different functionality, but it would be visible.
I don't know what sort of behaviour would occur when Microb/Conversation application overrides the arrow with it's own.