we recently made a base plate extension for our makita mortiser, we drilled the base plate and carriage bolted a wedge-shaped piece of wood to the base plate.
This changed the angle of attack, however, be carefull if you use this method, the clamp that holds the mortiser down has less power, because the mortise machine is held up off the timber.
Alternately, if you can't figure out how to cut a mortice by clamping the machine flat(or wedging the base), you could first screw or clamp a wedge of wood to the timber, then use that as a morticing base. When using the makita, make sure the machine is clamping solidly, grab it and try to pull it off the timber as a safeguard, it should be solid!!
-m.
