Short answer: Unless something new has popped up in the rules that I'm not aware of, dropships aren't eligible for stealth armor according to the construction rules.
Longer answer:
I've seen it added to aerospace fighters and small craft because an ECM offers some real benefits in and of itself there. Thinking in tabletop rules terms, ECM adds to the to-hit difficulty to target whatever it is. It's like how ground units use cover. Only it's in space, it's an electronic cloud and it's moving with the craft.
All dropships by default, have their own ECM "cloud." You could say they have some kind of ECM automatically, a fairly large field that extends well beyond itself and can be useful for providing ECM coverage to other things such as fighters.
So the first problem, is that adding a Guardian ECM to a dropship, I don't think is technically rules legal, but also becomes entirely redundant and so a waste of tonnage.
Second problem is the armor itself, the experimental rules note that if the target is in space and fires any thrust it receives no benefit. So the only way this benefits a dropship is if they don't move. That's not a great system, for dropships, going dark (not expending thrust) makes you a considerably easier target to shoot at, or even board. It's when you are under thrust that it gets much harder to hit you.
So engines off, coasting, stealth armor works, but the stealth armor benefit gets offset by the lack of movement and the easy To-Hit modifier. Engines on, you are moving, harder to hit, stealth armor not useful and you'd have more/better armor with a different type for more protection.
It's important to realize that in Battletech, stealth tech doesn't work like real world stealth. You can still be seen on radar, you aren't invisible, it's just more difficult to get a lock and hit you.
Stealth in Battletech has been made a niche thing, good for some things but not all. It's not just outright superior. We have an unarmed variant of the Cheetah designed to be a purely sneaky stealthy recon bird and coast through an area, drifting, uses its sensors to gather intel. If you deploy several of those along different flight paths, spread out, it's annoying but the opponent has to consider if its worth deploying interceptors to chase those individual ASFs down, especially if there's a larger battle raging, and the answer might be no, not worth it, they are unarmed so a commander decides to ignore them. That's the kind of thing stealth in space is good at. But while you'd probably be willing to risk a handful of ASFs on that kind of extremely vulnerable duty, you wouldn't be as willing to risk a whole dropship that way. That's a much juicier target, much more likely to be intercepted.
The dropship equivalent to that is most typically some kind of Q-ship like the Trojan or Mule Pocket Warship. Or a modified civilian craft designed to look like a typical civilian dropship but is really a high tech surveillance and reconnaissance platform that is scooping up sensor and communications data and basically acting as a spyship. We have seen examples of this in canon.
Outside of that, for fancy combat with lots of zooming around maneuvering or dropship acting as scouts you are better off having lots of thrust to work with. I'm talking like Achilles and Noruff level speed, with enough armor and firepower to fend off attacks by pesky light ASF interceptors and the few dropships that can catch you. But the sheer thrust and speed at your disposal makes it difficult to catch you. So everyone can see and detect you just fine, but you are racing at speed 8-12 and outrunning most of your pursuers while they shake their fists at you in frustration.
If the thought/concept here is a stealthy insertion vehicle (troop transport) remember that the BT stealth tech doesn't make you invisible to sensors, just harder to hit with a weapon, but again, only if you aren't thrusting, and to get the troops from A to B, you probably need to fire firing those thrusters and engines a lot. Just getting into the correct flight path to enter atmosphere requires a lot of thrusting and positioning. Not to mention the powered flight from jump point to planet.
So I hope that helps, and someone please correct me if I got anything wrong here. But the essence of this is I don't believe it's possible if you are following the construction rules by the book, and even if you tossed out the book and did it anyway, it technically doesn't offer a lot of value. Not thrusting in space makes you a vulnerable easy target to both hit and board, and stealth armor requires that you not thrust in order to add value.
