you can set the table of your bandsaw to the angle of the centerline of the stock and it will cut correctly for the cast. there will still besome rasping as you said, but it won't be nearly as much
it can be done with a non-tilting band saw table... set your stock on a dowel close to the butt plate so that the required angle at the center of the plate has you lift the forearm up, then as you start the cut, use the forearm to lever the butt up against the saw's pull so that the blade is entering the butt at right angles and as you move to the center of the plate, slowly relax the the forearm so the butt drops down the saw's table through the middle of the cut, and as you move to the other end of the cut slowly push the forearm back down to raise the butt so that your cut exits with the blade again at right angles to the to the stock. that way the entry and exit of the cut is straight across the stock's width and the center is cut square with the cast's centerline. with a good centerline drawn on the top and bottom of the butt, you can watch the angle in relation to the blade and get pretty close to what you have to have. it is an aquired skill from using a band saw a long time, but it works pretty good for me. i have also tried it with success by leaving the tilt lock loose and keeping the stock flat on the table, using the forearm to lever the entire table as i cut. these have obviously not been done for a MLstock yet by me, as this will be my first build, but i have done this type of thing literally since i was a teenagaer, steadily, so i'm sure it will work fine for this. the idea of having to do a little rasping or shaping after i cut is pretty normal for me and generally not even considered against the results i'm after. good saws, sharp tools and good files and knowing how to use them, make it all pretty much a matter of regular operation in my line of work.
yup, ideally you should have the tang shaped to keep the buttplate at right angles to the center of the bore while the tang follows the centerline of the stock. i realize there are none made that way, but i making my own plate. out of 3/8 x1-1/2 brass flat stock, mitering it together and silver soldering at the junction of tang and plate, then shaping the long vincent "tip" into the joined pieces and tapering the plate down to about 1/8 thick at the heel, where it meets the heel plate..file, file, file, file.......and then file some more! iv'e already got a buck made of the shape of the plate and the pieces roughed out, i did that last night. next step is to get the long miter at the comb to fit good so the solder joint is just a thin line.