Is it possible to force the homescreens always to use portrait mode, but allow apps to use landscape mode like normal?
if you use a alternative launcher, then yes. they have their own setting, while the system still has its own setting.
for the record, i'm rooted, using ADW, keep in memory, and compcache, and i get ZERO redraws. not taking it out of the dock, not opening the keyboard and then exiting an app. with LP, i was able to elimnate all redraws except for those 2 circumstances.