We never see anything on screen to be able to say one way or another, but I hope that the reasoning laid out below will be good enough to show why there probably aren't enough Trill renegades breaking the reassociation taboo to set up any kind of alternate Trill society or black market.
- Symbionts are rare. Only about 500 are available per year for joining, versus 5000 host candidates per year. ("Trill symbiont")
- The purpose of hosting a Trill symbiont is for the symbiont to acquire new experiences over the course of several host lifetimes. Reassociation with previous romantic partners runs counter to that purpose. ("Reassociation")
- Being a host is considered to be a tremendous honor in Trill society, and initiates must go through rigorous training and indoctrination. The Symbiosis Commission would undoubtedly dump any initiate who didn't show the appropriate respect for the purpose of being joined — assuming they even managed to become an initiate in the first place. ("Symbiosis Commission")
- Space is big. Really big. Therefore, there's an absolutely minuscule chance of unintentionally bumping into lovers from former lives.
- Even before the Trill became a spacefaring race, planets are pretty big places too. I would also conjecture that the Symbiosis Commission would not place joined Trill in places where Reassociation was likely to become a temptation.
- Trill hosts consider the life of the symbiont to be tremendously important. Exile means the end of the life of the symbiont.
- Trill symbionts are very long-lived compared to the life of the hosts, and presumably are interested in sticking around and experiencing lots of new things. It's possible an old symbiont with lots of life experiences wouldn't care about exile as much as a young one, but this also argues against any kind of alternate society, since the old symbiont wouldn't be around for much longer.
These last two bullet points can be seen as begging the question — after all, the question asks why Trill don't form an alternate society or black market to get around being exiled, with a side order of spreading truth to the masses, and I've responded that they wouldn't do that because they'd be afraid of exile and death. However, I think there are a few things that make these points more reasonable.
For one, the Symbiosis Commission keeps a pretty tight control on aspects of joined Trill society, and keeps secret that far more Trill are capable of being hosts than are allowed to be, even from the majority of joined Trill ("Equilibrium"). For another, the Symbiosis Commission is very respectable. Assuming that the Trill interested in reassociating and breaking from society discover more people are capable of being hosts than is normally thought, who are other Trill going to believe when it comes down to it: the respectable, knowledgeable Symbiosis Commission, or a couple of Trill who want to break a centuries-old taboo? The relative paucity of joined Trill to the population as a whole also means that a significant number of like-minded joined Trill would have to exist and band together to form an organization that would last.
Certainly, Reassociation is still something that could happen and has happened in the past. It is a danger, and Dax and Kahn nearly succumbed ("Rejoined"). However, since there are relatively few joined Trill, the joined Trill have been carefully selected and trained by the Symbiosis Commission, the probability of unintentionally running into former lovers is extremely low, it is highly discouraged from a societal standpoint so as not to happen intentionally, and from the general desire to stay alive, I conclude it is exceedingly unlikely for there to be enough joined Trill who are both willing and able to be exiles and form their own alternate society.