That’s a neat idea, likely no small task to implement though.
Well, I’m not too familiar with the specifics of this forum software (phpBB), but it would really only involve a few small-ish steps.
1. Creating a new usergroup (usergroupB, for example) which has the same “permissions” as the current “regular users” usergroup. Basically a duplicate of the main usergroup which the majority of forum members are currently in.
2. Moving all of the users from the original usergroup (that’d be usergroupA, then) over to usergroupB.
3. Editting the permissions of usergroupA to disallow the use of URLs by anyone in the group. (As all new users are automatically thrown into usergroupA upon completing registration, none of them will be able to post URLs.)
4. Setting up a small promotions script, if one doesn’t already exist in the admin area, to automatically bump users up from usergroupA to usergroupB after they’ve hit the 10 posts mark. (If such a feature isn’t present in the admincp, the basics of a script could probably be plundered from the part of the forum software which handles registration validations. There’ll be something in there which similarly bumps users up from an “awaiting validation” usergroup to the “registered users” usergroup.)
You’re right though, I wouldn’t personally want such a task on my own plate, I’ve given myself a headache just by writing this post (or maybe it’s the length of time I’ve been awake?), but compared to integrating the high scores with the forum I’d expect this to be a breeze.