Any tips on how to diagnose and/or what might be the problem?
What happens if you manually run the pg_dump command quoted above against a running 9.1 server, outside of the context of pg_upgrade? (Your port will be probably be different from 50432)
If that still crashes, What if you drop the --binary-upgrade option? The --format=custom option?
gdb doesn't seem helpful:
$ gdb /usr/lib/postgresql/9.3/bin/pg_dump core
GNU gdb (GDB) 7.5-ubuntu
Reading symbols from /usr/lib/postgresql/9.3/bin/pg_dump...(no debugging symbols found)...done.
You need to install the debugging symbols, which I have no experience doing under Ubuntu, but it should be easy to find instructions for. Or you could compile pg_dump from source, after configuring with --enable-debug.