After thinking a bit more about the case posted by R. Sholom Kass, I looked in Sefer Pitchei Shearim and found this example. It seems that the case posted by R. Sholom would be similar to the second illustration here, which according to the Pitchei Shearim is obligated in a Mezuzah. The only difference between the cases is that in R. Sholom's example, the left wall continues beyond the descending wall. This is a mitigating factor regarding a brocho, but I believe we usually put a Mezuzah mtzad safek when it is the left wall that continues and not the right one.
It would seem clear that in any case the two columns in the middle are to be disregarded since they are only to hold up the roof. So perhaps there should be one Mezuzah on the right-most column and affixed without a brocha?