the stegadon is a monster, so if i take it as a mount for a chief it's a ridden monster... but the chief replace one of the crew, who are untargetable. So enemies can or not target the chief in close combat?
From reading the new LM book, it looks like the chief can only be hit by randomised shooting. Remember though that if the steg dies, so does the chief....
From reading the book I thought that any character taking the Steg as a mount follows the normal rules for ridden monsters, whereas the normal crew are un-targetable.
Unfortunately he replaces one of the crew, but does not become a crewman himself. That would be nice, since it would allow him to not be targeted separately and also allow him to fire weapons on top using his BS.