Sgian Dubh
Posted a bit tongue-in-cheek I have to admit, and definitely not in the hand tool tradition. I've never before thought to try one for that type of sliding dovetail and similar types of joinery, but seeing your joint images and your question caused the idea to pop into my mind for some reason. I suspect a multi tool might be a bit aggressive. I have however found these Multi Tools can be remarkably accurate cutting across the grain used with care, and maybe one might have application to finish off the hard-to-reach corner after you've done the bulk of the work with a hand saw. I might even try my multi tool one day on a similar joint just to see if it's a completely daft idea ... or not, ha, ha. Slainte.