I don't think he will. Rafa, Djokovic, Federer, Ferrer, Del Potro and Berdych are all better than him on clay, as well as some others outside the top 10. His main problem is the movement and I don't think he'll ever be as comfortable on it as players who learnt to play tennis on clay.
His back problems have flared up two years in a row during the clay season, so he maybe won't want to play too much on the surface if it risks his Wimbledon run.
I would like to see him end his career with a clay title though - any title. As you say Sparkle, playing tournaments in the South American swing would be a good idea. He'd have a decent shot at somewhere like Vina del Mar, as top 10 players don't generally enter, though he'd still have to play good claycourters.
There will be some good clay courters in the South American swing, so it would be a risk if he were to enter, but if he does decide that he wants to prioritise clay, then it would be worth it. The media might try to hype up the significance of wins and losses, but with a bit of luck he could string together a few decent wins against players who are ranked low enough that he can beat fairly easily, whilst trying things out. If he can do that at the start of the year, and feel more at home on the clay, he wouldn't need to push himself so hard when it came to the main clay-court season, so hopefully stay injury free.
I'm not convinced that would be enough to make him one of the favourites for the French, but I definitely think he could improve his overall form on the surface if he were sufficiently motivated to do so. It just might mean compromising other parts of the season.
I agree the proximity of Wimbledon to the French may also be a factor, even if it's just subliminal. With a Wimbledon trophy under his belt, he may feel he can fully focus on the French Open without worrying if he could be compromising his chances at SW19. The extra week between the tournaments will help too.