My first choice, in your situation, would be to follow Mark's suggestion.
Yes, you can use a 6" power hand planer and hit the face multiple times. I have a friend who did this on all of his timbers (a hybrid house like yours). In fact, I have used my 12" makita hand planer on 14" and 16" timbers, taking multiple passes. It can be done and the resulting seam is either not noticeable, or quickly sandable (with a power sander and 80 or 120 grit) to that point. The sanded surface would not be as glassy smooth as a freshly planed surface with sharp blades.
I wouldn't try to feed those timbers thru something like a portable table planer. I have a 12" Dewalt of that ilk, and it does good to plane 1x8" oak boards. I have, on the other hand, heard a rumor that someone supported a timber, and let one of those table planers feed itself down the timber! I would have to see it to believe it. On my list of things to try someday when I'm bored.
-Thomas
