There are no known geographic concentrations of toki pona users. The toki pona census
shows people are evenly spread across the US, Canada, France, England, Germany, Russia, Italy, Mexico, Japan and the Phillipines. Toki pona as a skill is traveling via the internet, not via physical space, so everyone is so far apart.
If Sonja taught all her neighbors to speak toki pona, then there would be critical mass in Canada. If the toki pona meetup group in Arlington Virginia
goes anywhere it could get critical mass, too.
Esperanto has critical mass because globally it has millions (hundreds of thousands?) of people interested of which a few hundred are willing to spend a few thousand and a week of vacation time to travel so far. Toki pona is no where near those statistics. Sonja's number of 2000 people who are willing to buy a book is the largest and most scientific count of people interested in tp that I know of.
The most common places for conlang seminars are science fiction conventions and programmer's conferences. Esperanto congresses have lots of people who probably have heard of tp, but it isn't a good idea to promote tp at an Esperanto conference unless one is doing so in Esperanto and even then, it's kind of a stretch.
Toki pona isn't proven as a language suitable for conversation yet. I'm not sure I could carry on a conversation-- while I can write toki pona rather fast, I'm really slow to read it and my comprehension is a bit low. Toki pona is the only language I've ever studied that was easier to create than it was to understand.
And while I'm kind of lapsed in my Esperanto studies, I have to admit that Esperanto is better adapted as a language for a congress-- the languages promises to be usable for talking to foreigners (especially those whose 1st language is a PIE descendant) while traveling and I think it delivers on this. Toki pona was never optimized to be useful for talking to foreigners whilst traveling.
How would one draw up an Congress Agenda and Schedule in toki pona? We don't have an agreed upon way of expressing times of day!