You can specify -1 which means "don't filter based on viewer" so you get a list of all features on the map. In more recent Warzone versions you can specify ALL_PLAYERS to get a similar effect.
3.1 Beta 1
featureID
String
Optionally filter the list to features of a specific type as defined by their feature ID.
You can get a list of feature IDs from the first column in features.txt
In Warzone 3.1 RC3 and earlier:
If no parameters are specified, defaults to listing everything (no filtering).
If the viewer parameter is specified, defaults to listing nothing (everything gets filtered out)
In Warzone and above:
Defaults to listing all features (no filtering) regardless of whether viewer parameter is specified or not.