It should be possible to get a better idea where the filtering happens with a tool like tcptraceroute (possibly patched to use other segments beyond the default TCP SYN).

I haven't found evidence of extremely widespread filtering. Why would there be? The installation count is not that high. The potential side effects from uncoordinated port filtering could be quite severe. This isn't netkit's telnetd or Busybox. (I'm aware of Debian switching defaults, but that was fairly recently.)