Hi from this MTB noob! I have a problem with a bike I just got- I like to keep the seat rather high to and from the trail (like a road bike) but when I get to the dirt I want to lower it at least 4", more for downhill. Problem with my current post is that even after cutting it as short as I dare, I can't get the post to drop as low as I'd like because of the design of the rear suspension. Would a dropper post work as any kind of 'height extender' while still maintaining the minimum tube insertion needed for safety? Hope this makes some kind of sense. Thanks!
Unfortunately, a dropper post is going to be restricted by the same thing. Many full-suspension mountain bikes have seat tubes that don't go all the way down, which limits the total effective travel of the seat post. If swapping seat posts was easy, you could have two posts that you swap between but you'd really need a second saddle too because moving the saddle between the two posts would be a massive pain. And carrying a second seat post with attached saddle doesn't sound remotely practical. Unfortunately, I think your best bet is to get used to riding to/from the trails with the seat not quite as high as you'd like.