Little update:
The combat mode I planned is not possible. Well, that's not the whole story. It's totally doable and works like a charm but is very easy to exploit. You can enable the combat mode, log of and back in and you get the benefits but not the drawbacks. I played around with some ways to make sure that the stuff is not exploitable but I can only make it annoying to use, inefficient or exploitable. Not really promising in my opinion.
Though, what we can do with such a combat mode in theory is to make sneak attacks more viable by adding extra damage for each sneak dice. The most control we would get is by adding a custom on hit effect to the weapon but that is not really a solution I am happy with because OnHit effects are very taxing on the CPU. The other solution would be, that we add a bonus damage effect to the player character which would be based on the sneak dices. Not an ideal solution as well because that kind of damage would ignore damage immunity and sneak immunity entirely.
Another solution would be to bury the combat mode idea and to create custom feats instead. I might be able to re-use some code I wrote. For example a single hit with a multi for your sneak dices and an instant death effect if the target is hit from a specific angle. Such a feat could profit from Crippling Strike, Epic Precision and Opportunist. For example:
Crippling Strike: Reduced AB, AC and irresistible slow/knockdown
Epic Precision: 1% Instant Death for each Sneak Dice with a CR cap.
Opportunist: +X AB for the attack.
Brumm