Weird output from MAX3232
So this turned out to be just a MAX3232 artifact combined with a DB9 pinout error (for which I feel silly).  On DB9 the TX pin (2) is supposed to go to the RX pin (3) on the PC side (male connector) - however if you're hooking a male connector up to a female connector you still want to measure the corresponding pin 2.  So I was actually measuring the RX pin.  When I measured pin 2 I got the -5V/5V inverted clone of the 3V signal I was expecting.  So the signal pictured is presumably a spurious signal that the MAX puts on the RX line if it's not connected to anything.  I wasn't expecting that, so I got fixated on correcting the output, but really I was just measuring the wrong pin.