Would runoff voting be possible in future US presidential elections?

Yes I think it could be done as you say with out a constitutional amendment. The ballots could be changed to allow voters to select a fist choice and a second choice on the same ballot. The problem is we would end up with too many canidates on the ballot. All the canidates who lost in the primaries would run as independents. The current system allows us to narrow down the field to only two persons and really focus in on them.