Major bike manufacturing companies, particularly the ones with the money to sponsor World Tour teams, don’t do custom frames any more, particularly not out of Carbon as the mold and layup process is so expensive for a one-off item. [/broadest of broad brush strokes]
The nearest I’ve heard of a major bike comany doing a custom carbon bike, is the Specialized Roubaix; supposedly the size L model from the late 00’s/early 10’s was essentially a bike tailor made for Tom Boonen, and then since they had the mold, they just made and remade it a million times over to sell to the public.
If Dennis was riding in the 90’s he would’ve just gone to his favourite frame builder in Adelaide and asked them to do a steel/Ti/Al mock-up of whatever brand he was being paid to ride and nobody would know. Nowadays you’ve got the Caley Fretzes in every pitlane at every pro race telling all of YouTube that “these wheels say Joe’s Round Things on them, but they’re really the new Unobtainium X Pro unbadged.”