You can do it, but not through the Minecraft launcher. Open TextEdit and make sure it's in plain text mode. If it isn't, choose "Make Plain Text" from the Format menu. Then enter the following text:
cd ~/Library/Application\ Support/minecraft
java -Xms512m -Xmx1024m -cp "bin/*" -Djava.library.path="bin/natives" net.minecraft.client.Minecraft "YOURNAMEHERE"
YOURNAMEHERE with the name you want, but keep it in quotes. Now save your file, and make sure you uncheck the box that says "If no extension is provided, use '.txt'." Save the file as
launcher.command (you can call it whatever you wish, just make sure it ends with
Now in order to make this launchable with a double-click, open up Terminal and type
sudo chmod 777 and then drag and drop your file on to the Terminal window. Then press Return. It will prompt you for your password, which you should enter. You can now close Terminal.
The file will now be executable, so you can run it by double-clicking. It will open Minecraft in offline mode with the custom name. If you ever want to change the name, right-click the file and choose Open With > TextEdit. You can then edit the name and save the file, and it will use the new name.