You can't use the browser, it'll always default to looking for files on disk.
You can however start typing houdini paths in like /img/OUT_the_thing and it'll try and complete for you, and show you a nice peach colour if you get the path right.
You can't however use this path in this state.
You CAN however then put op: in front of it, and it'll work.
You MIGHT however find arnold won't like it, depends if the arnold plugin has been designed to work with op paths.
It's a rollercoaster of emotions and half-baked UI features!
-matt