Not a primary chain failure, but mine had broken just above the final drive sprocket ie in the crankcase behind the primary drive.
Not as a good as an 'invisible' weld, but a PO cleaned up the carnage in the case and made a neat little aluminium plate that fills the gap. This is glued to the crankcase. You can see it if you're looking ie anybody who has had this experience with a unit twin (!), but a reasonable compromise, and presumably didn't involve having to strip everything to pieces.