Hi Jim
I can think of one instance where we were dealing with 6x8 knee braces that used 1 1/2" ash splines as tennons. The mortise was a through mortise on the 8" face of an 8"x12" post. We removed most of wood using a chain mortiser, however the machine will only mortise 6" when its perpendicular to the timber. After turning the timber over and doing the other side the waste in the middle was removed with an electric chainsaw! It was little like chainsaw art in the middle of the timber.
I don't know how long an auger bit a boring machine will accept, but an 18" x 1 1/2" auger bit in an electric drill with the drill guide set to 45 would have made more sense....in hind site.
I haven't tried this but I also thought boring machines set at 45 might work well to cut rafter seat cuts that are not on the shoulder but rather further in on the plate. Just my 2 cents worth. I'm Canadian so that's probably 1.4 cents worth :p
Ian.
Gee, I hope this image thing works.