Can I make a suggestion here? Me and ArnimS have been using /media/mmcX/share/gamename for data files. User will most likely want the datafiles on either card. So:

use maemo-confirm-text in preinst to ask which card user wants
symlink something like /tmp/gameinstaller to card/share/gamename
make package install data files in /tmp/gameinstaller (i guess if the files are owned by user it doesn't try to chown them. Not sure so check the pingus data package)
remove link in postinst
Make game look for datafiles from cwd. Then use a script to launch the game that checks which card the data files are on and cd into it before running the binary. That way user could actually swap cards on N800 and the game would still work.
Finally make prerm delete the datafiles from either card.

Most of this work is already done in Tower Toppler package. You would only need to add that symlink thing.