I wanted to set up a short URL for transmission-daemon on my NAS so that instead of awkward http://nas:9091/ I could type something more finger-friendly (most notably, without the colon for port number), ideally http://nas/t/. Here is the final config that resulted after several hours of research and
RewriteRule /t[/]?$ /transmission/web/ [R=permanent]
ProxyPass /transmission http://127.0.0.1:9091/transmission
ProxyPassReverse /transmission http://127.0.0.1:9091/transmission
Thanks to everyone on forums and StackOverflow for numerous solutions that allowed me to piece this all together.