While still a bit of work, I just use the duplicate function while in dynamic placement mode.
I click on the random scale/rotate buttons, then click on an already placed tree (or other asset), and instead of left clicking to place, I hit ctrl-D for duplicate........this will place a tree in the spot, as well as keep another tree queued up.
Move a bit, ctrl-D, move, ctrl-D, move, ctrl-D (don't left click or else you'll need to select the asset again). Keep a decent distance, and grab a different tree and you can fill in more gaps. Once you have a nice section going, grab em all and duplicate the whole bunch and slap them nearby. Don't forget bushes and other forest materials and can do this same process with any asset.
Auto ground snap set to True, though dynamic place will snap it there - just a precaution if you choose.