[UPDATE: Actually, I like this better: http://360percents.com/posts/command-line-twitter-status-update-for-linux-and-mac/ -- a simple bash script that also could be used with AppleScript if you really know what you're doing. The method below may also work.]
Another way to do this (if all you're after is automating the process of tweeting) is download twitchi, which is a command line tool for twitter, and write an AppleScript utilizing do shell script. You could still have your twitter client open.
Admittedly, I haven't tried it yet, but I kind of want to. If you are interested in how this would work, I'll test it. Here's the output describing usage:
usage: twitchi
-af,--addFriend Add friend.
-au,--auth Authenticate twitchi to post to your Twitter
account.
-bu,--blockUser Block user.
-dm,--directMsg Send direct message.
-h,--help Show usage information
-m,--msg <arg> Message/Status text/Search query. To be used in
conjunction with other options.
-p,--page <arg> Page number. To be used in conjunction with other
options.
-ph,--proxyHost <arg> Proxy host. To be used in conjunction with -sp.
-pp,--proxyPort <arg> Proxy port. To be used in conjunction with -sp.
-rf,--removeFriend Remove friend.
-rp,--removeProxy Remove proxy.
-s,--search Search.
-sd,--showDM Show direct messages.
-sf,--showFolls Show followers.
-sp,--setProxy Set proxy.
-sr,--showFrnds Show friends.
-st,--showTimeline Show timeline.
-u,--user <arg> Username. To be used in conjunction with other
options.
-ui,--userInfo Show user info.
-us,--updateStatus Update status.
-ut,--userTimeline Show user timeline.
-uu,--unblockUser Unblock user.