You can cut a angled mortise with a cross-grain mortiser, by putting a wedge of the same slope underneath. It's pretty easy.
Definitely do the diminished haunch as Tim mentioned.
Laying out slopes on a timber that's not perfectly square can get pretty weird. With that wedged half-lap you have three sloped surfaces--the top of the wedge, the bottom of the tenon, and the top of the housing. What I do is lay out everything on the primary reference face (where the wedge goes in) first. Then draw your slopes on the two side faces. Then connect where those three slopes hit the inner non-reference face (arris to arris). You can only square them across that face if the timber is perfectly square. If the timber is not square, then the those lines will angle across that face. This will be most obvious on the diminished haunch, because it's the steepest angle. If that's utterly confusing, I can draw something up in SU.
