[Fisneaky]

Well, this ones not possible due to the model ,except for a "Composite" maybe as a "prop" (as in properties) for movies.

Checking the settings for Alpha channels in the .mat. some background objects disappear others don't.

Could do with a reference section for the .mat files once things are known for sure.

Heres what I've observed so far:- If you know more please pitch in.

[General]
tfDoubleSide 1 // none observed
tfShouldSort 1 //applies shadow from mat
[Layer0]
tfBlend 1 // 0= no alpha
tfNoWriteZ 1 //transparent to background ob
tfDepthOffset 0
tfTranspBorder 0
tfTestZEqual 0 //1 makes invisible - shadow only