EFF mentions contacting "the embassy in our country", I would assume they mean the Embassy of Syria, in this case. Unfortunately, I don't think that Syria has an embassy here in the US, as its operations were suspected last year.
Can we in the US do anything for Khartabil?
Edit: Actually, I see that the US officially recognized the opposition a few months back and that they will (or have) opened an embassy here: http://www.bbc.com/news/world-us-canada-27287650
However, the article said that Khartabil's prison is on the front lines, and evenly split between the forces. Which side of the prison is he on, I wonder? I'll need to do additional research. In any case, my original question stands, which is what exactly can we do, what tone should the letter take, etc?