I was doing this myself. However many tablet users are not skilled in linux.

It would be possible to make a .deb package that moves /usr/share/games to mmc and makes a symlink to it. However this could affect other applications and is therefore bad policy.

Ukki and I chose a more 'conservative' solution - i.e. one that does not break convention of the root filesystem. The game is launched from a shellscript in /usr/bin which passes the data directory to the game, and the postinst script modifies the script to point to mmc1 or mmc2 depending on where the user chose to install the game.

For very large game data files, I modified this process to make the binary .deb postinst set up a symlink to 'share' for that game only, then when unpacking the game data goes to mmc without consuming space on root filesystem.

This requires some additional packaging work per-game, but I think it makes the least assumptions about the target system and is thus 'safest' way to achieve the goal.