Here's the behavior I'm imagining: you double-click on a note in the piano roll and, holding the left mouse button down, begin scrolling with the mouse wheel. This cycles through various chord types, all using the note that you've clicked on as the root, and places a preview of the other notes in each chord automatically. When you release the mouse button, the chord that you've scrolled to is left in place. Moving the mouse cursor while previewing just moves the currently displayed chord to a new place.
I think this would work best if the possible chords were arranged a little differently from how they currently are in the drop-down, so that, roughly speaking, the simplest or most commonly used chords are near the beginning of the list, and more and more extensions are added as you move along the list.
The next time you engage this feature, the chord type that you decided on the last time you placed a chord is the starting point. In conjunction with the reworked list, this means that if you're sticking with major and minor triads, you'd only ever have to move the mouse wheel one step to get the chord type you want. But if you're making something jazzy, you'd scroll up into the seventh chords once, and thereafter only scroll a few steps to get whatever variation of a seventh chord that you want.
Also, it'd be neat if, while you're holding the left mouse button down and scrolling through chords, holding Alt and scrolling cycles through inversions of whatever chord you're currently looking at. Releasing Alt and scrolling again sets you back to the first inversion.
Also, right-clicking while previewing a chord could play the chord so that you can hear how it sounds, and each chord type would have a floating label that's visible until you release the mouse button.
I think that a feature like this would enable someone to put any chord progression, even a very complicated one, into the piano roll in maybe a minute with very little effort. A simple C-G-Am-F progression might take ten seconds.