Ouch... I have a Cort C4P with a similar problem, so I've been thinking about this.
I think the best bet will be to [b]carefully[/b] remove the jack and all the electrics from the cavity and make a carefully-shaped piece of aluminium or steel plate to put in the cavity, then glue and clamp up the fragments and the plate so that the plate supports the fragments from behind, if you get my drift. Obviously the plate would have to be drilled to let the jack pass through.
Problems with this I can see are:
- the bits may fall out and might be beyond reassembly. I think that photos and slow-drying glue should sort this.
- for this reason, the jack would have to be held tightly while unscrewing the nut which holds it.
- I think it would be wise to earth the plate.
- there may not be enough space in the cavity for the plate (especially since it's conductive - the electrics would have to be separated from it).
- the cracks will probably always be visible if you look closely.
I think it's probably a luthier job, TBH. I'd get an estimate first, personally.