Okay, so I guess there'll be a better way that people use on this one, but bear with me because I'd like to know if my thought train is going in the right direction and thinking in the right way ;-)
A picture is worth a million (or less than ten) words:
Basically, reface your bookmatch, aligning the end grain so you know it's totally face to face (mark the edges to align?) and clamping the wood in the direction you want to angle the bookmatch. Using a jointer or router table, rout along the guide and Bob's your mother's brother!
Obviously, issues such as tearout can be dealt with by good bits, not routing too much at once and dampening the edge slightly I guess.
Would this work well, or just be a potential waste of good wood? It seems to work in theory, and you obviously learn by your mistakes and experience. So I've decided to learn from others mistakes and experience rather than from my pockets bad experiences.