I'm a big fan of occasionally chucking my players into the middle of a situation - my experience is that it produces memorable and enjoyable gaming sessions, so long as it's done with care.
In an upcoming campaign, I'd like to introduce the PCs during a pitched battle (towards the end, but the players won't know that) and I'd like to add some kind of mechanical effect to emphasise the environment & description. I've had a few ideas, but I am concerned that anything I do removes choices from the players, one of the cardinal DMing sins.
To compensate for that, I have been thinking about balancing what I'm taking away with an extra benefit, like "you've already used an encounter power (pick one) but you've earned an extra action point" or "you're bloodied but you can use two Second Winds this encounter".
I'm not really a mechanics-focussed kind of DM, so does this kind of thing sound workable? Is there a better way?