<drumroll>OK, the magic formula for making backdrops from scratch (like the Dark Star backdrop):</drumroll>

1. Build the model (use a size similar to that of the stock backdrops), using the MTM1 transparent texture.

2. Select all faces

3. Goto "Groups>Set group magic number" and set it to -2147483648 (Strange number? No way! It's the smallest possible signed number that can be stored in 4 bytes)

4. Save as Backdrop (this will not recalculate the magic numbers, thus preserving the -2147483648).
 

If you need a more thorough explanation of bins and their file format and how they're used in BINedit, please go here.

- Thank you Oliver Pieper.