Ok, so, you know how sometimes Home Depot will surprise you by what they don't have? Sometimes, although less frequently .. they'll surprise you with what they do have.
I found this by serendipity last night:
It's meant as a vent cover for 2" PVC, sits inside a coupler piece.
So, a quick bout with the chop saw and the top of the standpipe now looks like this:
So I cut a new piece of pipe (only had about 12" left of 2" PVC) and extended the standpipe. Looks like I could have gone 1/2" - 3/4" longer still, but nevertheless it's still a huge improvement in noise:
http://www.youtube.com/watch?v=wed0uKr0AY8