Seems like a NAT issue with static outbound…

[jmcskixc]

Well, the packet capture was very enlightening.  It showed, well, no packets at all.  A little detective work with alternate ports indicates that my lovely ISP is blocking port 80, despite allowing port 25 and being a commercial connection.  Changed to another port number and everything works as it should.  As for the original issue with the VNC ports, I suspect there was a separate issue there as well.  I'll consider this mystery solved.  Thanks for all your time and effort, it certainly did lead me to the solution.  Port forwarding works just fine, as long as the packets actually get there!