Due to time constraints, we need to launch a specific experience. An email goes out, if the user has the app, the link brings the user directly into the app. If the user does not have the app, it goes to a download page (we do not yet have the mweb or desktop experience ready). We want to put copy in the email that alerts the user to make sure they open it on their mobile device with the app installed. Has anyone seen functionality out there like this? Given this isn't the best UX, what is the best practice for wording?
It's very likely that no matter what, a certain percentage of your users are going to try to open this 1) on an unsupported device, or 2) on a supported device without the app previously installed.
I think both cases are fine, just design for that workflow - the user's next step is to get on mobile and download the app. On desktop, you could show a beautiful landing page outlining the benefits of getting the app; maybe the user could click a button to have the app store link sent to their mobile device by SMS.
On mobile, you can provide buttons to the respective app stores, and a smart app banner that links directly to the app in the store on the user's device.
(Code implementation is off-topic on UXSE but I promise that smart app link banner code is extremely easy to implement, and information is easy to find.)
Once the user has the app downloaded, perhaps you can highlight your new feature on the welcome screen.