dropper seatpost in Cabin Luggage

You'll possibly get pulled aside to explain what it is, but it's not a weapon or anything. Maybe talk to the TSA at the airport before you get in line for the security check. 

Let us know the Air Carrier - could be some CSR's out there who can  help you.

I would take all of the air out of the post before you bring it on. I don't think the change in cabin pressure would cause it to explode or pop open, but better to be safe about it. Also, some airlines are very picky about tires being deflated on bikes in cargo so they may pull that rule out for bike related things, though I have never had to deflate my suspension while travelling.  

I put my axs post and derailleur in my carry on and have never been stopped or questioned