I remembered this overnight. Manfred Moore, an NFL Super Bowl player, was recruited by Newtown RFL team in Sydney in 1977. He played 4 games, scoring in his first appearance, but ended up in reserve grade before suffering an injury that prompted him to return to the US and the NFL. I remember seeing him play, but it seemed that it was mostly a publicity ploy. Wikipedia page:
Manfred Moore - Wikipedia
It is worth pointing out that the stamina bit really comes into play here. Sevens, unless I am very much mistaken, is seven minute halves where as union has forty minute halves.
That’s a huge difference.
Umpteen years ago I played rugby with and against some former pro and semi-pro American-style football players and I also knew a couple ruggers to try out for NFL teams; as others have said the two games really don’t relate well in terms of how someone would do. A NFL lineman may have some hopes of being a good prop or lock more than a back being a good rugby backfield but that is just opinion on my part. This is all for 15s ------ 7s really didn’t come along until after my playing time.