Turin,
I discussed this with the FAA yesterday...they do not allow any exception to 629. The 737 upper wing frost allowance was, apparently, based on an allowance by Transport Canada for West Jet (there may be other authorities involved, I'm not sure). The FAA was asked to look at it, but turned it down flatly because they cannot identify a reliable method for the evaluation of upper wing frost, i.e., 1/8 inch or 1/4, how can you tell by looking out the cabin window at night...
Frankly, I think they're spot on. A failure to detect upper wing ice during the preflight inspection is pretty common, particularly around the wing root area which is not visible from the cabin windows. I'm not sure how anyone can accurately assess a frost presence beyond determining that it is, or is not, present, and even that determination is not always reliable...