by Andrew Nicols.
I've just been discussing this with someone on IRC. It looks like Moodle doesn't currently support OAuth message signing with x-www-form-urlencoded. It currently only supports application/xml style messages.
See http://www.imsglobal.org/lti/v1p1pd/ltiIMGv1p1pd.html#_Toc309649688 for more info on the specification. There's an example in http://oauth.googlecode.com/svn/spec/ext/body_hash/1.0/oauth-bodyhash.html#anchor15
Andrew