EDIT: my, my, we are impatient. I believe I said "later this evening". No? Perhaps my "hav(ing) gone mad" has affected my sense of time.
--------------------
O-10, eventhough I am not a cynic by nature, I seriously doubt that the "major war is over"; but, I always welcome a cease-fire.
Re you question, and please understand that I am taking your questions at face value. If you meant something different by any one of them, I have no way of knowing:
A well known jazz artist would not go to St. Louis and then decide that he wants to gig. Unless, of course, he goes to spend a summer vacationing with O-10 or other personal reason and then decides he wants to gig. A well known jazz artist would generally go to St. Louis AFTER he or his booking agent has lined up some engagements in clubs or festivals. Simply showing up in St. Louis does not guarantee being able to get gigs at all; even when well-known. Clubs and festivals have artists booked and rosters filled WAY in advance.
Now, lets assume that the scenario IS that the artist decided to hang with O-10 for a few months and then decided that he wanted to also do some playing while in St. Louis. A "gig" is a paying job and he might be able to get some gigs in small clubs on short notice by virtue of the fact that he is well known. These gigs pay very little. The artist may also simply want to play (for no money) to keep his chops in shape. Being well known means that he probably already knows who the best players in town are, or knows someone who does. By contacting those persons he can find out if there are any scheduled jam sessions in town or can put together a trio for an engagement that he may have been able to book. If the artist goes to St.Louis because his agent has booked some gigs for him, the agent may also book the other musicians by contacting booking agents in St. Louis or hiring the players that the artist wants to play with based on recommendations by other players or personal experience.
I’m not sure I understand what you mean by "improvisational jazz, no Moonlight In Vermont or other standards". If you are talking about "free" jazz, where EVERYTHING is improvised and there are no "tunes" or predetermined structure to the music, then "gigs" will be just about impossible to get as there is a very limited audience for this kind of jazz. It will also be more difficult to find decent "free" players. If you are talking about the kind of jazz that is discussed here where there is typically a "tune" (melody) and then the players improvise over the chord changes of that tune, then "Moonlight In Vermont" qualifies as much as any other tune that has been posted here. It has a melody, a certain form, and a set of chord changes. It is an appropriate vehicle for improvisation and, in fact, has been played and improvised on by many jazz artists. As recently posted, there is no reason that Sonny Rollins playing "How Are Things In Glocca Morra" should not be considered "straight up jazz".
The community of jazz musicians is a small one and players know each other or know about the ones who can play. I suspect that the clip of Eddie Fisher playing live is a perfect example of a player showing up to play with a band he had never met before. This has been discussed before. While the idea that players who had never met can play so "coherently" is impressive, keep in mind that, especially in the case of that Fisher clip, it’s pretty formulaic music with very simple chord progressions or even one single chord for all the improvisation. Most decent players can do this in their sleep; and , in fact, the players accompanying Fisher are decent and no more. Now, to play "Giant Steps" with its complicated chord changes or to play simpler tunes at the highest level of artistry is a different matter altogether and only a well rehearsed band can do that credibly.