There's no real precedent. Senators die all the time and governors appoint their successors. Most recent case being NJ Senator Jon Corzine getting elected governor and appointing his own successor to the Senate. At last report, Corzine and Bob Menendez were not married to each other (Menendez won election outright in November, despite widespread allegations of massive corruption, which NJ voters evidently don't think is as serious as sending suggestive emails to congressional pages).
Big issue here isn't what happens if Johnson dies; that's pretty clear-cut - the governor appoints whomever he wants. But apparently, South Dakota's state constitution doesn't provide for replacing a senator in the event of his disability.
Could raise interesting issues. As long as Johnson remained senator, the Dems would still control the Senate, with the biggest advantage being that they would have majorities on all the committees, and the committee chairmen would all be Dems, so they would control legislation. If it were to drag on for months, with Johnson in a coma, Repubs would then start asking - very delicately - if the Dems were properly the majority party any more, since one of their members had not carried out his duties.
This could be fun, if you can get past the tragedy of a man lying unconscious in a hospital bed for months at a time.
