You can cut an angled mortise with a cross cut mortiser. If you use the guide that references the face parallel to the depth of the mortise then you take your first plunge on the angle of the taper... mark the angle to the offset of your guide. If the mortiser references the face perpendicular to the depth of the mortise then you can cut a wedge to plunge the tapered angle. It is similar to using a drill guide from your drops for a piece of threaded rod at a rafter to kingpin to rafter connection.

If the wood is green you can always put the wedge for the dovetail on the inside to then tighten after shrinkage. I prefer it from the outside, if any force pulls on it will pull the wedge into the joint. If you are enclosing the structure from the outside there is no need to nail the wedge... the enclosure should hold the wedge in place.

With the makita mortiser I would recommend using a wedge on the high side of your mortiser... don't just rely on the clamp to hold the mortiser in place. I like to cute a few triangles made from 3/4 MDF (what was on hand in the shop at the time).

This is one of the best tension joints that can be used in wood joinery in tension. It has been pegged in history... I don't see the need other than to hold it in place during the erection. The wedge should hold it in place for the rest of time after the erection.

William Denton IV