[Filter plugin] Can we add a new plane for transparency?

Hi,

I created a filter plugin and I'd like to add transparency to the image. If the image is already a layer, it has already 4 planes (1 for transparency) and it works fine, but if the image is flatten, there are only 3 planes and I can't find a way to add a new plane to add transparency.

I can disable the plugin when there is no transparency (with the PSHOP_HasTransparencyMask property) but I think it may be confusing for the user and automatically adding transparency would be better.

Is there a way to do that that I missed? I tried modifying FilterRecord but when I try to switch to the new outPlane advanceState function returns an error.