histogram equalization based on a pixel region

I have a sequence of grayscale images that were taken in different sunlight conditions. In the sequence, there are no moving objects in the upper left corner, so I would like to set the histogram of each image such that the upper left corner is constant brightness.

For two whole images I am able to get the histogram of image B to match the histogram of image A by

hgram = imhist(A); 
B_adj = histeq(B,hgram);

Or I could make the entire B_adj match the histogram of the upper left corner of A by

hgram = imhist(A(1:100,1:100)); 
B_adj = histeq(B,hgram);

But I don't want the histogram of the entire image B_adj to match the histogram of the upper left corner of A. I know imhist offers an optional grayscale transformation array T, but I don't know how to use it. Any hints would be much appreciated.

[Chad Greene]

I'm still looking for a solution to this one.