"but it struck me as something of a post-hoc rationalisation" I'd kind of agreee with that. It is a hell of a misreading of a song even for a mixed up character in a novel to make. The song is about wanting some sex with someone you meet in a field of rye (isn't it?) whereas the character imagines he's saving kids as they are in danger of falling over a cliff from a field of rye. The only thing that seems to connect them is the use of the word rye!