While in theory anyone with a kilowatt output and good antenna system could control access to a single channel FM sat by instituting some form of net procedure in practise nobody has done so.
What I beleive people want to have are random QSO's that they initiate themselves and that are aren't controlled by anybody else.
If you get more than half a dozen Amateur operators active in the footprint of a single channel FM satellite they'll inevitably be congestion issues.
We'll just have to live with that for those satellites that have already been designed, but for future designs a linear transponder seems the obvious choice. A 40 kHz linear transponder would allow for multiple SSB QSO's over areas with a high Amateur population but would still enable the use of FM in the many areas of the world where there are few Amateurs.
73 Trevor M5AKA