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That would be one of those jobs where even if I could (I could, but only really badly) I wouldn't want to. It would need to be back-purged, 'Cos Ti, and so would require one-off back-purging fixtures. And you'd have no idea what tubing was used in the first place and whether it was up to the job. If I was tempted to do the job, I'd make sure I charged more than the price of a replacement frame so I didn't end out of pocket.
Is there anyone near-ish London who can weld titanium, specifically a disc tab onto a Ti frame?