I travelled across Libya recently on a transit visa, but in the other direction; Tunisia to Egypt. I applied at the Libyan consulate (not the embassy) in Tunis and it was ready about a week later. Talking to other travellers I met, entry requirements seem to vary depending on your nationality. Some have to provide a letter of explanation and get their passport translated into Arabic whereas I did not. Translations can be easily done in Tunis though.
The visa gave me a week to travel (without a guide) from one side to the other. It was easily extended for a 2nd week in Al Khums and I should think in Tripoli and Bhenghazi also. I was told a 3rd week would not be possible. I didn't leave the route "across the top".
In theory it should be same going the other way, I did meet one couple coming in the other direction, but I couldn't tell you about the application procedure in Cairo.
Hope this is of some help,