A workaround can be that you write your message on some online clipboard service and post that link to Twitter.
Now this is very cumbersome and secondly, your twitter friends will have no click the link to see what your message is all about.
A practical solution to this problem is Twitzer - a Firefox extension for Twitter that will short tweets that seem to cross the 140 limit.
Type your “extra long” tweet in the Twitter box and shorten with a simple right click. Twitzer truncates only some part of your message and replaces it with a link to the full message.
This Twitter add-on is from the same guys who developed ShortText.