I think that's an awesome idea! Most of the time I come into those sessions on my phone on the way to work (no, I'm not the driver...) and Slack would be a better solution for mobile than what the campfire currently offers. I also sometimes forget when it's on (usually when it's 8am- 9am AEST) and with Slack you might be able to push me a reminder ?? Maybe??
I'd also be interested to hear what people who've been the UXpert in those sessions think.
You would have to figure out what to do when people send their questions via private message in Slack to your UXpert rather than in the channel itself but I think it's doable.
I vote we try it out, see what happens and then go from there