It is a pain, you would have to submit your own versions of the android and iOS app, bundling your FCM info. I'm not even sure they would be accepted since the app is already on these stores. It might be impossible to do this.
edit: for Android at least, you could give your users a modified APK.
Maybe somebody could build free versions of Zulip apps that use UnifiedPush instead and ask the server to send pushes to their preferred push notification provider. (Of course, this would mean you have to keep both apps and the server soft-forks updated with upstream changes, but it doesn't seem too hard of a project.)
From the FAQ: "iOS doesn’t support running services in the background, so running a UnifiedPush distributor won’t be possible without jailbreaking or Apple’s approval for the foreseeable future."
I think a better strategy that would require less ongoing maintenance would be for someone to build the mobile app and a routing server similar to what is in place now rather than reengineer the core server and mobile apps to use a different tech stack for notifications than it currently does.
I kinda feel that while doing something like would serve a segment of the user base this would also be a bit disingenuous to the folks at Zulip who have done an amazing job at balancing commercial monetization needs with their dedication open source principals and giving away their labor of love to world in general.
It has been a while since I looked into it, but don’t both app stores have provisions for offering private apps available only to people within an organization? Still seems like kind of a pain to maintain release parity, but not out of the realm of reason for an org that cares a lot about keeping their communications fully in-house.
1. Apparently, only one MDM is allowed per iOS device. This is a no-go for BYOD Consultants (like me).
2. Apple Business Manager does support Ad-Hoc distribution of links to private custom apps, but is limited to only one country. I have at least one client that has consultants across countries (typical when hiring in EU). The client doesn't even have a business registration in each country, so they can't even sign up for Apple Business Manager per-country they have users in. And even if they could, that would cost 300 USD per country.
Android doesn't even need the app to be distributed via Google, or even that an app uses Google's Push servers.
Publicly available apps for orgs that require an account in order to do much of anything exist. You could download the app and end up in no-account purgatory
Push notifications can be cobbled together for self-hosted Zulip using self-hosted Gotify and letting users install the Gotify app. No requirement for an app store approved app. However, Gotify is blocked on iOS via lack of viable background connections.
edit: for Android at least, you could give your users a modified APK.