Jeff Miller has a design for a mortising jig that I made and I have
found incredibly useful in a number of situations. Basically it is
just a 30" long block with a trough on one side that captures the
router's edge guide and you clamp the piece to the other side. He
wrote a description of it in a FWW article a few months ago. He also
describes it his book on chairmaking and I think he describes it in his
book on beds.

It seems to me that you could use that jig with a few wedges to make
angled mortises.

Mark