Can I publish a "coming soon" version of my application? (e.g. information regarding the upcoming service, ability to reserve a spot)
As long as you're releasing a functioning app that works, I can't see a reason why it wouldn't be approved.

However, this assumes that the app actually serves a purpose in its current state and that all you're doing is including additional info such as Coming soon etc.

If Apple does have an issue with the current version, you'll receive an email from iTunes Connect advising you that the App Store Review team have sent you a message and you can view and respond to it via the Resolution Centre. Typically this gives you the chance to address their concerns and, if necessary, to make changes.

Even if they didn't give you this opportunity, you can always resubmit the app once you've updated it.