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I think if the Ti tubes are vaguely close to a standard diameter then finding a length of carbon tube could be easier, if it's custom dia then it'd be a case of getting one made £££ or a slightly slightly larger tube being sanded down. Old Alan/TvT frames have been fixed by tube replacements like that but they might be flexier (plus conventional skinny tubes i guess)
Either way, I don't think you'd necessarily have to de-bond the entire frame but no, probably not cheap.
That's actually a fucking good idea, and probably cheaper than getting a speciallist Ti welder onto the job.
Well done that man!