When an Object Mask Tag is added to an Arnold Procedural, the object mask aov does not create any result. It seems as if it is a problem for the Object mask to work on an external file/object that is not in the scene?
It's a limitation, object mask can not be used with a procedural.
Object mask is actually an aov_write_float shader added to the root of the shading tree, thus it would require to modify the shader network inside the procedural, which is rather complicated.
Any creative ideas on the best work-around for this? Cryomattes seem to work with procedurals so that's one way to go, but it's a lot of manual work if you just want a mask in Photoshop... Is there a better way mayhaps?