Protocol for establishing longitudinal meridians on other heavenly bodies
As an example consider the prime meridian for Vesta. 

Based on Hubble images, the IAU had established a prime meridian based on the observation of the Obler's regio (a dark region). The prime meridian was defined as passes through the middle of this region. 

When the Dawn mission arrived at Vesta, it was discovered that the asteroid's pole was not in the same place that it had been assumed to be (based on Hubble data) and that Obler's regio was not a convenient basis for a prime meridian, as it was not even discernable from up close. Instead they chose a small crater, Claudia, that was close to the equator, and defined the prime meridian based on that (the prime meridian is defined as a passing 4 degrees from the centre of Claudia) This is now the generally accepted coordinate system, although it has not been "officially" adopted by the IAU.

In general, if you are the first to map a new world, you get to choose where do draw your meridian.