One way to get around this that I thought of is to maybe not depend the mod on number of frags but rather base it on time. You could try beating the demo'd player to the frag. Not being able to make the frag before the demo'd player did will result in penalty points and beating the demo'd player to the frag will add points to your score [the earlier the frag, the better the score]. Time will be reset every time one of the players in the original demo is fragged. Maybe this would need to involve custom demos, like a campaign mode.
I'm sure I am overlooking something here so that this is impossible to accomplish but since we're brainstorming here
