Correct, you need to build the profile upon a DNG and the use the profile on raw files (DNG or camera raw files). The TIFF is rendered and the profile is out of the picture at that stage.
I tried the OP's approach some time back, though in trying to use the flat field plug-in. Finally dawned on me that that didn't work either.
I know we DP Merrill users are a very small user base but I sure wish Adobe could support them. Now that the DP M Quattro is about to be released, I have little hope of DP Merrill support for my current Merrills.