Hi DL, I would think a transportable band mill would be good for this task. I cut a lot of clapboards for all my siding needs. 1/2"x4" in lengths up to 20' at times. It is easy to cut thin stock from a larger cant. The trick is the last piece, as some mills don't cut down to 1/2", I hold my breath at 3/4". So the last board is cut at 1-1/8" and I leave the last 1/2" cut on it and flip it over so I can cut the 1-1/8" board in to two boards. I will usually gang up 3, 4" wide heavy planks which I have cut out of some nice clear white pine logs, removing most of the sap wood. The heavy planks are cut to yield the most vertical grain as possible, kind of quarter sawing but not in a true sense.

Just thinking that is a lot of stock to hand feed through a up right band saw.

Could you get some nice butt logs in your shipment of pine you are searching for?

What is the exposure on your shingles?

Shingles and clapboards need not be tapered.