I do stop motion with Lego and when I want to make something fly, I make a video with the character supported by bricks and/or clay, then I remove all the pieces from the set and take a picture; then, using GIMP, I import the blank image of the set and paste a frame with the character supported by bricks on top of that image and erase the bricks which shows through to the blank set image creating the impression of flying. I have been trying, for a few weeks, to do this in Blender to speed up the process and have not been able to find a way. How could I this in Blender?
There are two and a half options.
1) If your support bricks are in the EXACT same location in every single frame, you can create a mask in the compositor, and use the VSE to render out the result. The mask could be used to composite in something else, or just make the masked area transparent. It depends on what you need.
2) If your bricks are NOT in the same place, but are a different color from every other thing in you scene, you can use a chroma key to create a mask of everything that's a particular color.
3) This is the half-option. If your bricks are NOT in the same location, and they're NOT the right color for a chroma key... you might be out of luck. However, if you're SUPER lucky and the tracking is good, and you're SUPER good at video editing in Blender, you could make a mask that tracks the edges of the brick, and use that mask to make the bricks transparent, or white, or whatever you need.
Any one of these would make a HUGE answer. Detailing all three of them is far beyond the scope of this answer. BUT there are good resources out there.
Some good searches are
blender alpha mask video
blender chroma key
blender camera tracking mask
Searches are often difficult when you don't know what TERMS to use. Other times, Google is just overly specific. These searches seem to give good results.
I hope that helps!