I like the idea, but the geometry is tough. You will be approaching the planet at a significant speed, even if you're willing to take a while, and that speed needs to be shed somehow. You can probably get the burn time down into the hours, which is much less warning to the defenders than days would be, but it's still nonzero. And you can't just kick the mechs out the door as you fly by, because that kind of speed would almost certainly result in them burning up in the atmosphere.
You might do better to see if you can use a moon to mask your burn. If you're in the moon's shadow, you can burn fairly freely, and get close at a fairly high coasting speed while not actually retaining that speed all the way down to the surface.
I'm not a physicist, so I'm not even going to pretend to try and use the correct language here. What I'm describing should work though. So it will be easier for me to explain, instead of thinking of it like a planetary orbit, let's think of it as a clock.
Take a normal wall clock and lay it on its back. The clock is the solar system, with the tip of the hour hand being the planet you want to land on. The center of the clock is the sun. Your ship is going to start at the zenith point of the solar system -- meaning you're basically "above" the center of the clock. If we start at noon, then the hour hand is pointed at the 12. If it normally takes 1 hour to go from zenith point to planet, then you would want to aim at the 1 o'clock position. You would accelerate until you're halfway there, then you turn around and decelerate until you arrive at the 1 o'clock position. You'll be moving very slowly, relative to the planet. There's probably some sort of adjustment you're supposed to make to match speed with it, but that is not described in the fiction. This is the normal way people do it.
Instead of doing it this way, we are going to try and be stealthy. We are going to aim ourselves at the 2 o'clock position. The "planet" will get here in another hour, so we've got time to do things unconventionally. We accelerate towards 2 o'clock, cutting our engines at the halfway point, just like normal. But instead of turning around and decelerating, we are just going to coast the rest of the way. Now, when we cross the horizontal plane of the clock (passing through where the hour hand will be in the future), now we turn around and we decelerate. We are traveling "below" the plane of the clock. We slow down while facing away from the planet. Once we hit zero velocity, we keep accelerating and then we spring back up, like a yo-yo. We then arrive at the 2 o'clock position at the same time as the hour hand. We have never had our engines pointed at the planet during the whole trip.
I used to do the same thing when I was playing Asteroids as a kid. Accelerate one way, then turn and accelerate the other way at a slightly different angle. Except here we'd be using real math.
Now, we wouldn't actually accelerate to the halfway point, just like we wouldn't actually aim for the 2 o'clock position. The exact measurements, speeds, and angles would be made by a person with a physics degree and not a guy who majored in history in college. And it will all be complicated by what angle drive plumes can be detected by a planet. This factor is unknown to us, but the people flying the ship would know it. Final adjustments to match speed with the planet would make it tricky, but NASA does that stuff right now with probes that don't have mega-powerful engines.
