VICTORY! I finally found the name of the plant I was looking for!
The Latin is Mussaenda philippica. A picture is at this link.
Finally, the mystery is solved, and I can rest easy again, except for
the snippet of a tune I heard through the window of a passing car 5
DAYS AGO that I have yet to be able to identify, but one mystery at a
time... http://plantsdatabase.com/showpicture/16230/