That chamber of commerce deal sounds very reasonable - when we were going we recommended it was done by a bloke in London who was linked to the Libyam consulate in some way, who wanted £80 per passport to do the translation.
I sent my passport to the passport agancy, and they provided the stamp (which just says name,occupation, date of birth etc in arabic), and sent the passport back to me by recorded delivery free of charge. I then went to a local language school and got one of the arabic teachers to fill in the details for £20.
There were lots of stories floating about at the time that there had to be official stamps, it had to be done by certain people etc etc, but at the end of the day all that mattered was the guys on the border could read the passport - to be honest they barely did that - I think it was the swiftest border crossing I have ever done. We had more hassle leaving Tunisia than entering Libya. (by the way, don't fall for the Tunisian border guards telling you to hand over all Tunisian currency before you leave the country..tuck it away and play dumb.)