Red, now it's your turn to figure out how to do it with an S shaped convex and off-axis joint
I welcome your input Wojtek. It's easy, just fire up the CNC mill
Or, as some of my Navy SEAL acquaintances used to say "there is no problem which cannot be solved with the proper application of explosives".
Let's look at it another way. No pre-shaping of the stem (or anything for that matter). A surface of a briar block is treated in some manner to form what will be the first mold line. Now, in an entirely separate operation a contrasting block of material (ebonite, acrylic etc.) is prepared and one surface has the identical mold line as the briar so that the two will mate together perfectly. Keep in mind that the mold line is contoured, not straight. Glue the two together. Now, by whatever means available, the exposed end of the contrasting material is altered in such a manner as to follow the first mold line leaving a uniform insert/inlay that is say .156 thick. Again in a separate operation another block of briar has one surface prepared that will in turn perfectly mate to the insert material. Glue'em together and now make your pipe with whatever method you prefer and with whatever stem cross section you prefer. The methods employed to generate the contoured mold lines are whatever floats your boat.
Please note that the above statements were produced with the B.S. generator from George's link. Also George, note that I carefully avoided using the word "machined" and instead used the verbs "shaping", "treated", "prepared", "altered", "make" and "generate".
Perhaps George is right and I have been dealing with the DoD for too many years.
Oh well, TGIF!!!!!!!!!!