So I'm in the process of attempting a repair like this on the top of my Shortbass. I do have a hole with a separate piece of wood thats actually detached mind you. My plan- gleaned from youtube- is to stabilise the area by glueing bits of veneer across the split from underneath, clamped with rare earth magnets whilst they dry. Then spread a lavish amount of glue over the veneer, and drop in the wood that has come away. Leave to dry.
Then, make some similar colour filler with some PVA and sawdust, and fill the cracks. I daresay it won't be perfect- especially with my woodworking skills- but I have hopes it will be playable once more, as these don't seem to be easy to get anymore.
Good luck!