The original question is an interesting one, and I don't really know the answer without checking my reference books, but it's much earlier than 1976. The first certain reference to Robin on crusade that I know of offhand is the novel Maid Marian, The Forest Queen, by Joachim H. Stocqueler (1849). Certainly in the second of the De Koven light operas about Robin Hood (1901), he is off on Crusade, which may be the first real pop-culture reference. And of course Robin Hood goes to the crusades in the 1922 film with Douglas Fairbanks. And R. Padgett is correct (other than the typo); in the 1950s Robin Hood TV series, Robin was a returning crusader, although not that much is made of this during the series' long run.

On a related note, Robin Hood was first placed in the correct PERIOD to go on the third crusade in the 1521 Chronicle known as the HISTORIA MAJORIS BRITTANIAE. This is a great double-entendre title, as the author is one John Major, and the title translates from Latin both as "A History of Great Britain" and as "Major's History of Britain!" In the earlier chronicles, Robin was stated to have lived later, in the 1260s or 1280s, while the earliest ballads place him in the reign of "Edward," but don't say which one--presumably Longshanks!

As previous posters have noted, in most of the versions of Robin Hood that place him in Richard's reign, it is precisely the period during which Richard is on crusade--and Robin isn't--that most of the action takes place.