i suppose that in order to prevent the problem with GradientMergeMosaic you could "black out" the area in the wider field image that corresponds to the narrow-field image. to do this, i think it might be easiest to make a copy of the narrow image which has been registered to the wide image, and then apply the pixelmath iif($T>0,1,0). that should create a mask where all the pixels that have data in the narrow image are set to 1. then mask the wide image with that mask and apply the pixelmath 0 to the wide image. save that as a new file, and then run GMM on the 'blacked out' wide image and the original narrow field image which was registered to the wide image.
if you get artifacts you could experiment with removing some wavelet layers from the mask to smooth the edges a little bit, or maybe use morphological transformation to shrink the mask down so that you still have some overlap between the two images when running GMM.
rob