Does Android block any ports when tethering?
The answer to this question is that no, neither Android nor my carrier are arbitrarily blocking traffic on high-numbered ports.

The problem is that I had forgotten how paranoid I am about security and that, in addition to making SSH listen on a non-standard port, I had also created an iptables rule to only allow connections to that port from my home IP address and my work IP address. Obviously I was using a different IP address when attempting to connect via my phone, hence the failure to connect. (And I probably didn't look hard enough when prompted by @Izzy to check server logs, because I'm sure there is a record of the blocked connection attempt in there somewhere). D'oh...

My apologies for wasting people's time, but in case anybody else thinks they have a problem with Android or their carrier arbitrarily blocking ports, hopefully this answer will prompt them to triple check everything about their connection, including their server firewall, before they go off logging tickets.