Figured it out after stewing over this series of commands a few minutes and experimenting...
I had installed Composer as a “global” install on WebFaction so I could use it, if necessary, on any webapp I might install in the future, as opposed to just Flarum now. This means the composer.phar file is located in $HOME. So, turned out I needed to create an alias to that file in order for it to work from the Flarum install directory, which the WebFaction guys didn't tell me originally.
These two commands were all I needed to run after tunneling in:
echo -e "\n# Composer\nalias composer=\"php70 \$HOME/composer.phar\"" >> $HOME/.bash_profile
source $HOME/.bash_profile
On WebFaction, you have to make sure the right version of php is indicated, equal to your domain's webapp type set for it in the WF dashboard.
