Anything's possible in theory, but that doesn't mean that there's much more than a snowball's chance in summer of "overcoming" a system that is determined, by bi-partison agreement, to squelch third parties. This is why candidates and potential candidates of the calibre of those you mentioned are very much unlikely to take the third party route.
The reason, by the way, that John Anderson made it into the debates was that Ronald Reagan insisted upon it, and Jimmy Carter (as I recall) declined to debate in one of them. Perot was invited by the other two candidates, I think at the insistence of one of the major candidates, either Clinton or Bush. Neither Anderson (who was an independent candidate, not a third party candidate) nor Perot would have been included by the debated commission's rules alone. Since the time of Perot, none of the major party candidates have been willing to have a three-way debate.
Put this alongside the several other factors that guarantee third parties will be of slight importance in general elections--and some factors that I didn't mention--and the truth is that the door is basically locked and has remained so after decades of court cases brought by one or another third party.