Kouja Dai
We might wait and use Silence's idea - infiltrate one of the queen's castles, attempt to claim a hostage. I can't see either side making much headway so early on, so it could go one of two ways: the rebels confront a queen and/or her dragon, fail in kidnapping, and escape OR succeed, take her/the dragon for ranso, but the kidnapee escapes. Either way, there's some confrontation involved and the royals are armed with true knowledge of a rebellion.
The idea can occur via an infiltration, OR (perhaps more likely) the rebels wait for her return from the mountain meeting and confront her/the dragon on the road at night. Those are my thoughts thus far.
This may seem odd, but, if decide to kidnap a dragon, then Gaul is the... less dangerous choice. He seems more docile than the others and (though this is not yet known) He has decided to up hold his Queen's values. (i.e. Allow himself to be captured to save civilians)