I don’t think Loeb should be pigeonholed as a pure rally one-trick pony. His racing experience has been multi-disciplined, even if you discount his wins in the ROC. He’s competed at Le Mans in 2005 and 2006 (finishing 2nd overall) and was pretty successful in minor GT campaigns on the side.
Given his pace on tarmac and experience in wheel-to-wheel racing (as opposed to the point-to-point WRC style), I would expect him to be decently competitive in an F1 race and it’s a pity that his age will likely prevent us from seeing him do a reverse Raikkonen, even if only for a single race.