Yeah...I would have had to do some kind of encryption, or at a mininum, encoding of the data stored in the .sav file to prevent people from cheating.
If you recall the name of the other homebrew that has this capability - please let me know - I'd be interested in seeing it.
Re: the leaderboard
I am looking at enhancing the leaderboard to add some new responsetypes. I plan to add, in the near future, a method for the ds to retrieve the top 10 scores without registering a score to allow "browsing" of scores. Maybe I could add a type that places your highest score in the ranks of the online leaderboard, but doesn't register the score. So you could at least see what position you placed right on your ds.
Overall, it may be a good idea to look at encryption and building a sample project so that dev'rs can encrypt their save file...but not sure how feasible this is....cause I haven't spent any time on it....