Oh come on Arkaelis with your legendary abilities we could figure this out. Just because the peons of history couldn't do it, doesn't mean we the chosen few, can't figure it out
I actually think 4e has given us the chance to actually do this because criticals are so much less powerful than they have been in the past, at least from my experience so far.
Okay so -10 is too much. Great. -2 is okay. What about in between. If -2 for +3 is okay, then can I give a -6 to hit and then get +9 to damage? Would that work?
If so, then if I required this to be a light weapon, the differences for getting a critical wouldn't be huge. Let's say for sake of argument I made it a basic attack only which means it is always only 1W, right? Then correct me if I'm wrong but the most damage a light weapon can do is d6, so we are talking about the difference between an average of 3 damage and 6 if it is a critical. That's nothing. Then of course we should assume a magical weapon, make it +2, which gives us an extra 2d6 (average of 6). So for -6 to hit, it looks like to me I get what I want with the restrictions that this is heroic tier, usable only with a light weapon, and a basic attack. What am I missing?
If I made this a feat, then you have to expend a feat to use it, so I would think that it would be okay to make it -5 to hit.
I think we could tweak the above mechanism to include sneak attack too if we wanted too. We could also tweak it to allow more than just a basic attack by making it usable less often.