Example of using imregtform to optimize about a specific area of image (only)?

I've got some simple motion tracking that already gives a pretty good estimate of the transformation of moving relative to fixed. ...which I pass as "InitialTransformation" to speed further refinement.

However, I'd like IMGEGTFORM to only focus on a certain part of the image (as explosions are literally happening in other parts of the image, even very close to the desired area of focus).

None of the optimization metrics had such an option (disappointing!), although, it seems I could crop the moving image around the area of focus, and use IMREF2D to provide the details of that crop (i.e., as "Rmoving" argument) to IMGEGTFORM, and have it fit only the cropped portion.

Is this the right approach? Any examples out there of doing this?