Idea for a mass combat scenario, seeking advice

I've got a scenario coming up in a month or so in my game which will have the PC party visiting a dwarven fort while it is besieged by gnolls. I've read through a bunch of different mass combat systems, but they all seem overly heavyweight to me. I've come up with what I think will be a fun light-weight way to run the scenario, and was wondering if anyone had any ideas or feedback to offer.

The pcs will be assigned one of the main gates of the fort to defend. They'll fight 3 or 4 separate encounters of the usual size, against small waves of enemies -- gnolls, demons, and some interesting monsters they've impressed into their army, such as a gauth. The terrain will shift over the course of those encounters, as enemy sappers cause cave ins and collapse walls and such. Between each fight, allies will relieve the PCs, so they can fall back and take short rests to spend surges and regain encounter powers.

At the same time, I'll be showing the larger battle on a separate map, at a larger scale, with minis representing platoon or company sized units on both sides. I'll use simple opposed d20 checks (skill checks?) between units to resolve the mass combat, once per round.

During their fights, the PCs will be able to use minor actions to make skill checks, to relay orders to various units on the large map. So for example, an Intimidate check to whip up a unit to fight harder, a Heal check to send combat medics to back up a unit that's taking a pounding, or a Bluff check to order a unit to feint and flank an enemy unit. They'll also get a chance to use one minor each for orders between each fight.

So, any ideas or advice on how to improve this? The one part of this that still feels a bit weak to me, where I'd really like some tips, is how to resolve the unit-vs-unit fights, as simply as possible. I definitely don't want to go with detailed hit points and attacks for units -- I'd like something lighter weight than that.

The 3rd edition sourcebook, Heroes of Battle, has some great advice for mass battles, and its all encounter design/philosophy, so it applies just as well to 4e. The gist of it is that you run a mass battle like a dungeon: just a series of separate encounters that the PCs have, with their outcome affecting the overall tide of the battle, which is just a series of events that you narrate. If the PCs do well, you describe how they are helping bring victory to their comrades. If they screw up, tell how the tide has turned against their forces. If you wish, you could have the outcome determined by dice rolls, with bonuses or penalties based on the PCs actions.

I dont know if this is applicable to you, but I have been contemplating the same thing and wondering how to handle.
Bit of background 1st. Back in 3rd edition days I tried a big battle...it went for weeks, and didnt really keep the players interest the whole time. I wanted it epic, and made it too epic : Too much to do, too much to keep track of, too much to go wrong. So building on that I came to the conclusion that I didnt want to try to emulate a tactical wargame again.
Rather, this time around, I am choosing to represent the battle as a series of skill challenges (Note : We have a kick arse skill challenge system with supporting software ) whereby I will define what is going on in the battle with possible events. Players can choose how to contribute (someone might want to use a heal skill in the triarge, someone else might want to work on inspiring the troops, someone else might want to get into the thick of it).
Based on this dynamics are defined (e.g. How many troops remaining at the north tower?) and influenced by player skill contribution. Triggered events then lead to further skill challenges (e.g. The number of troops on north tower gets to zero...north tower overrun) or even(/inevitably) to pre defined (limited scope) combat encounters. For instance, troops cant really cope with the trolls smashing through the front gates, its up to the heros to seal that breach.
From my position, the point is NOT that troops are litteraly represented as markers on a map, but that the battle is followable (i.e. the current state of play is transparent to the players) and transitions from state to state, and that the players have a real and understood ability to contribute to the result and influence the outcome.
I figure this way I can represent a battle and still be true to the game the 4e actually is.