Advice on offering free DB sites

What I plan on doing soon is offering databases sites up for free. Now going in the same lines of directory scripts that do this the only think I am looking for in return which will be to keep 2 footer links on the site and not to remove them.

Now the database site is very valuable if you ask me. Just image being able to download and instantly have a site for lyrics, articles, baby names, famous quotes, jokes, ect. Thats a damn good deal and all I ask in return is you keep the footer links.

Now the question is does anyone have any suggestions on how to enforce this?

The only way I can think of and I think its a damn good idea is to mispell a word or change a word to another somewhere buried in the database. So when I do a search for that phrase I will pull up all the sites that are using my database and I can check for backlinks. This would be effective because it would take people hours and maybe never find the one little change that I made.

But I was wondering what you all thought and if you have any ideas.

Thanks