I think I remember a Cubs(?) reliever being put in LF for one batter then brought back to the mound sometime last season. Would that be the same basic situation?
The comment to Rule 3.03 says that you can do that, i.e., put a pitcher into a position and bring him back, but only once in an inning. HEre's the official comment:
A pitcher may change to another position only once during the same inning; e.g. the pitcher will not be allowed to assume a position other than a pitcher more than once in the same inning.