It would seem that a non-definitive pronouncement could not be infallible, by its very nature as non-definitive. This is clearly shown by the fact that such pronouncements require only religious assent (cf. LG 25) and not a definitive assent - the submission of faith.
Although the individual bishops do not enjoy the prerogative of infallibility, they nevertheless proclaim Christ's doctrine infallibly whenever, even though dispersed through the world, but still maintaining the bond of communion among themselves and with the successor of Peter, and authentically teaching matters of faith and morals, they are in agreement on one position as definitively to be held.(40*) This is even more clearly verified when, gathered together in an ecumenical council, they are teachers and judges of faith and morals for the universal Church, whose definitions must be adhered to with the submission of faith.(41*) (Lumen Gentium §25)