The answer to your question is yes and no.
No typically you can only designate the pitcher if line up cards are exchanged. In play off games line up cards are not required. If you designate a line up card you are required to pick a designated hitter for the pitcher.
If your line up card has a mistake in it, and the umpire tendering the cards does not catch a mistake of a picking what hitter is the designated hitter, then the designated hitter can not be substituted for the pitcher. They would be substituted for any other player in the line up that is not the pitcher.
Combination of rule 4.03 and Rule 5.11
http://mlb.mlb.com/mlb/official_info...cial_rules.jsp
Post script....both leagues CAN use a designated hitter, per the official rules.