Fred, Maybe someone else will have something more to your liking but I've never known anyone that didn't do it by hand. The amount of material that needs to be removed is usually fairly small. Accurate measurements, careful marking and a sharp chisel then sandpaper strips to fit are what I use.
I have to agree Ned, by hand is the best way. Many use a router and jigs but I say you can't beat sharp hand tools and accurate measurements.
I made such a fixture years ago, and it worked - sort of. Problem was that it took ten times longer to set up, calculate, measure and cut the heel than it does working trial and error by hand. Everybody I know uses the sandpaper strips, too. . .
A tip for flossing your cheeks... (fitting the cheeks to the sides...) is to stick clear packing tape on the back of your sandpaper strips and they will resist tearing much better and last longer.
After a lot of stumbling with chisels, I've settled on using only sandpaper strips to take the heel angle down. Sure, it's slow, but there's a larger "safety factor" in doing it slowly. The only time I'll use a chisel these days is to undercut the cheeks adjacent the neck tail which, as Hesh notes, cuts down the area that needs attention.
The MicroMark mylar product is good, and I also use cloth-backed strips of abrasive rolls in different grits. The current best-bang-for-the-buck comes from Harbor Freight, they sell a box of 4 different grits in 20ft lengths for $10. They're strong but tend to be on the thick side, so the mylar (polyester, actually) from Micromark gets used for the final fit.
I've had great luck with the giant rolls of cheap sandpaper at Harbor Freight. It loads just terribly, but it works quite well.
If you have undercut the wood by the tenon, you can then use a chisel to remove the bulk of the wood up to the line you need to cut to, then finish off with the sandpaper. Don't be too aggressive with the roughest paper, you can tear out some wood that way.
I've made a wooden caul from a piece of scrap maple that is flat on one surface and curved on the surface that goes against the vise jaw. It allows one to hold tapered parts nicely. Also I often use a piece of 1/2" Homasote on each jaw to prevent dinging up the part. I have a "luthiers" vice with padding glued to the swiveling wooden jaws. Here's a pic of the caul for a plain old carpenter's vice. It is low, though (better seated at it), and the bench mounted luthier (with pivoting jaws) or parrot style vices are easier on the back.
". . . the underside of the heel clamped between a flat planing surface that would guide the cut."
"Between" implies two objects on either side of a third. Between a flat planing surface and what? Without a photo, I have no idea what you are talking about. (Although I have noticed that often others seem to need less information than I do before giving an answer to a question.)
I THINK the OP is referring to the flat area that would define the new scribe line for the new neck angle, no? And that guides a chisel along the new scribe line..