Unless I’m missing something which is possible, don’t you just need some fancy corner bosses square but wider than the coving with a nice finial end pointing down. That gives you something to butt up against without having to do mitres. Make them out of wood with the fancy end turned in the lathe.
I do have a fancy gauge somewhere, which I’ve never used, that has two arms which you clamp pieces of cove to and then present to the wall. Digital angle gauges give you a readout so you can set your table saw up to do compound angles and it cuts the ends perfectly. That one only works with flat ceilings though.
The alternative is scribed joints.
One other trick that looks quite nice is to cut the ends in a scallop with the same curve as the cove so as to leave the top edge longer. This works when you run up to something that isn’t being coved like a cooker extract. Much better than just butting up which looks less finished.
regards Martin