Acquire Function in the UVW Map modifier should allow you to pick any polygon to create Planar mapping.
I do a lot of modeling where objects are required to have Planar mapping. One challenge is aligning UVs from one object to the next. If you add a UVW map to an object and try to match the UVs of another object (using the Acquire) function, it will only work if you try to select an object that already has a UVW Map Modifier on it.
That is problematic because the target object may be Unwrapped in such a way that you don't want to apply a new UVW (which will mess up the existing mapping).
Or, add a new functionality to the already proposed UVW Mode in other threads here. In that environment, it might be nice to be able to easily transfer mapping from one poly to another (with options for relative/absolute like there is in the existing UVW Map acquire function).
Shawn Olson commented
It's obvious that this feature request isn't very popular! For anyone who is interested, though, I did find a plugin that does let you Acquire UVW from arbitrary faces in the scene as well as give you a ton of other useful controls for easy Planar mapping: PolygonMap by Vladislav Gavrilov http://www.vg2max.spb.ru/PolygonMap.htm
I highly recommend this plugin.