I have a text file with about 400 signatures and quotes. And today I finally found an easy way to insert one random signature to my emails.
The SignatureProfiler plugin for Mail will let you add quotes & iTunes status, HTML links & images, and "Tail Signatures" for multiple accounts. You need to format your text file and configure the plugin before use. Read the "About" tab in the plugin for instructions.
I like to use John Gruber's Non-Top-Posting Reply Scripts and RapidoWrite to type my standard signatures and contact information.
That means I still have to click the drop down signature in Mail.app. But it chooses a random signature every time! How cool is that?