For a while now I have been using Gimp masks to create vectors, but just today I had a photo of a finished piece that would need a lot of work to create a mask. So I thought about how I created the masks to make vectors and thought “Hey! we could import the vectors to make the masks”. As the vectors made the piece in the first place it was merely a matter of scale and slight rotation and I had a pretty good mask!
i’m not really sure i follow your process, but what i do when i need a “clipped” bitmap is open it in illustrator, create the shape i need it clipped inside of, then use the clipping mask in Ilusttrator, and then i rasterize that clipped bitmap at the exact size i want it and make it grayscale and with DPI to match the LPI i want to engrave at. rasterizing it throws away the clipping mask and chops off all the pixels you want hidden.
Now I understand what you were saying!