In the military (UK) each daily flying boss nominates the unit/types' alternates and base ops normally contact the nominated alt and make an arrangement, sometimes resulting in refusals when an alt has reached theoretical parking capacity if they all diverted. A nominated alt would then remain open and honour their agreement even if their own base flying had ended unexpectedly.
In civvy world when a flight plan is addressed a copy is sent to the alt(s) by AFTN, so in theory, provided the AFTN address is actually read by someone, they will see the FPL and you would hope, remain available for it! But in the commercial world there's no guarantee of either!
The captain of course has responsiblity for a legal alt, and that includes whether it is published or NOTAM'd open - after that there will always be an element of fate, hence it is wise to have a couple if available. As far as I know permits and political relationships fall to the way side in the event of a plane needing to divert, but that doesn't mean the people meeting on the ground are that generous and forgiving.