Is it possible to modify gstreamer used by matlab to specific functionality?

I am working on a problem that requires me to access motion vectors from an H.264 video stream for the purpose of motion detection. I currently have this working using the JM H.264 reference software which is written in C. Problem is for prototyping this is pretty painful and time consuming. Any time I need certain feature, I have to write the algorithm and code from scratch. For example, to perform blob analysis to detect objects, I have to write the code from scratch vs. Matlab where it is a single line of code.

What I would like to do is modify gstreamer to provide an interface that delivers motion vectors, then recompile gstreamer and have Matlab use that new interface. The data can then be manipulated by Matlab and overlaid over video using standard Matlab functions.

Anyone aware if this is doable, or has been done? Any tips would be greatly appreciated.