I would have thought that a path that I defined in $ROS_LOCATIONS
would be usable in a launch file with the find directive i.e.:

<node pkg=foo name=bar type=bar args="$(find mylocation)/my_file.txt">

.. however this doesn't seem to work; should I just be using plain old
environment variables for now, and/or could roslaunch be extended to
behave the way I'd guessed it might?

I guess another way to put it is, why/where to use ROS_LOCATIONS
rather than environment variables?

Cheers,
Pat