One of the interesting stats that came out of the 2015 Qatar Open was that it was the first time since 1971 that a reigning World Champion (Magnus Carlsen) had played in an Open Swiss. Of course "Open" in this case wasn't exactly Open, as there was a lower rating limit on the field.
The previous reigning World Champion was of course Boris Spassky, who played the 1971 Canadian Open. This was indeed a real Open, with the tournament field ranging in strength from 2441 all the way down to unrated. Curiously Spassky did not have the highest rating in the field, this hnour going to Duncan Suttles (2441). In fact Spassky was one of the "unrateds" in the field, as the tournament crosstable was published with Canadian Chess Federation ratings. Spassky did in fact win the tournament (like Carlsen), although he tied with Hans Ree (on 8/11), with Spassky's last round win enabling to catch Ree, who drew his final game.
Apart from these two players (Spassky and Carlsen), I suspect not many World Champions played such events during their reign. This has less to do with hubris though, and probably more to do with the fact that Swisses were not that common before the 1960's, and up until the 1990's Round Robin events were still the most common events for GM's to play in.