Hey Adam!
There's no plans for this today. Giving all users that form of access makes things complicated, because now they need access to parts of the administration UI.
That said, assuming we're talking about Review Board 3.0's Slack support, then it's very possible to have an extension just simply provide a UI for creating a Slack configuration and then to create the necessary database entries to do it. A Slack configuration is just an entry in the integrations_integrationconfig table that provides a configuration name, an ID representing the Slack integration (a static value), and then a JSON dump of the settings (basically, just a channel name and the conditions under which it will post to that channel -- review groups).
It'd be a pretty easy thing to put together, and we could help with some sample code on constructing the right entries. You wouldn't even need a form as fancy as the one in the admin UI. Just something with a couple fields that can be exposed to users. A few hours of work.
If we're talking the legacy rbslack extension, then there's nothing that can be done there, as it was never built to allow for more than one mapping. That code is dead, with the version in Review Board 3.0 being the way forward.
Christian