It seems to be on-device only but if you have an Apple silicon Mac, you can create a partition and install Sequoia 15.1 there, then use Xcode 16.1 to build to "Mac - Designed for iPad" (so, Catalyst) and you'll then see at least Writing Tools if you have Apple Intelligence enabled in System Settings. And you can also try out Writing Tools from Safari and Notes and presumably other apps on Sequoia.

I'm on macOS Sequoia 15.1, running in a partition, and it's now twice gone back and forth between Apple Intelligence being available and then reverting to saying "Joined Waitlist". How can one get off the waitlist and have Apple Intelligence stick around?

I have not been able to get the tools to display at all. I’m curious which Xcode beta, which simulator and which Sequoia beta you’re running. Do you see the tools in all text views, including SwiftUI ones? TIA for anything you can share about your setup.

If you're seeing this, as I was, when trying to use git in Terminal, you may need to install Xcode command line tools. I needed to do this for Xcode 14 beta 4 and now again for beta 5. In Xcode, choose Open Developer Tool from the Xcode menu (first menu), then choose More Developer Tools... This will send you to the Apple Developer site, where you can download what you need.