Demo is a rather ambiguous term.
There are many hacks labelled incomplete for a reason.
That said I imagine it would be a case by case basis going by end results on the game most of the time. Say you hacked a game to double damage and were tweaking the drops and enemy placement a bit as double is a bit too much for all but the truly hardcore players, however still had the last 4 levels or something to do and just wanted some feedback on the what you had done. That would probably fly.
If you still had a bug to squash at some point but it still allowed some useful play then again it would probably fly.
Similarly we have seen many games rendered far more playable by a menu hack (usually puzzle or fighting games).
Maintaining a thread on the forums is probably the better plan for a lot of things but there is scope for things.
For the most part the curation is there to make sure things are usable, and that the database does not get flooded with "I changed the starting pokemon and the ones on the title screen" type of hack, and the equivalents for the other games with tools.