Hi Derek,
You can use the planar projection modes if you use a Transform Input Shader to convert the texture coordinates to world space, enabling "use world space" and putting the transform shader somewhere downstream of the image map shader or the default shader.
Here's the longer explanation: The planar projection modes don't automatically switch to world space, they're still working with the texture coordinates which are defined by your UVs, although the shader thinks they are 3D coordinates rather than 2D. If you're were working on an object where the texture coordinates were generated from world space (e.g. the terrain after a Compute Terrain node) then the planar projection modes would do what you expect. You can generate new texture coordinates using a Transform Input Shader with "use world space" enabled, by putting the Transform Input Shader anywhere downstream of the shader.
The reason it doesn't automatically use world space is so that you can affect them with warp shaders and other transforms which only modify texture coordinates. Perhaps the image map shader would be better with two versions of each of these projection modes.
