I tested datum-label support with the following expression:
(let ((x '(#1=(a b) #1))) (eq? (car x) (cadr x)))Gauche, MIT, Bigloo, Kawa, Chez, Ikarus, Mosh support the syntax and return #t;
Racket, Chicken, Scheme48/scsh, Chibi (in native mode), SCM, Larceny, Ypsilon, STklos, Scheme 9 report lexical syntax errors;
Gambit, SISC return #f for whatever reason.