If memory serves, Greg LeMond never raced on a bike of his own design, he didn't have his bicycle line/company until either after he retired or at the tail end of his career. If Lance wanted a line of bikes and be a subsidiary of Trek I'm sure he could easily do it.

Besides what does having raced a frame of your own design have to do with how it would sell in the marketplace? Didn't Lance have input on various aspects of the bikes he raced with Trek? His input was certainly used on the last two series of bikes that he raced, plus the helmets, shoes, etc.