mIRC is not capable of encoding on this level needed for OAuth moocat . The problem with mIRC is the same one people encountered with VB6 when trying to apply hmac encoding to a (string,key) Because there is no native hmac function, conversions must be done between hexidecimal and ascii, in order to complete the encoding.
Sadly, what you have encountered is mIRC's inability to leave certain ascii characters alone even when you tell it not to touch them.

There are some solutions to this but they vary depending on your mIRC version and Windows version.
Sorry but I am so VERY tired of dealing with this, that is why I made the .dll . This is why I don' t take this project any farther. I just don't have the time to field all the mIRC related issues.
If the .dll is working for you, I would just use it. If not you can find me on irc.geekshed.net #Script-Help .
I can make you a script to call a php routine from mIRC. (You must have php installed on the same machine with your mIRC for this to work) .

So to clarify what I'm saying. The problem has nothing to do with the syntax for creating the OAuth signature, it's the inability of mIRC's $sha1() identifier to process certain ascii characters.