Actually if you are registered for News upload (even via web news upload not FTP) you have 600 characters for Caption and very specific IPTC data requirements (completely different to stock). I use the web news upload frequently as I haven't asked for FTP yet but I am registered for News - see My Alamy for those that haven't registered.
News guidleines including metadata: http://www.alamy.com/contributor/help/news-picture-guidelines.asp
Aha thats why, I applied, but so far didn't received the response, thank you for letting me know. I will wait a little bit more