As a progress report, I have added relevant information to our FAQ:
https://fedoraproject.org/wiki/QA/Phabricator
and we linked that wiki page from Phab landing page:
https://phab.qadevel.cloud.fedoraproject.org/

I'm not sure if we can disable custom email registrations. It would be great if we could at least patch the login screen (the one with Login or Register button) and put there a big info that you should use your @fedoraproject.org email address.

I'd love to see the process be easier but it's not trivial to do that in phabricator.

phabricator requires a local user account, regardless of which auth method is used - we only have persona enabled but there are plenty of other options. I limited it to @fedoraproject.org email addresses because that is how ipsilon/fedoauth ties persona to a specific FAS account.

I'm open to the idea of allowing non-FAS auth, though. It made sense at the time but I'm not tied to the idea of 100% FAS auth for phabricator.

OpenID isn't happening unless someone else writes the code and gets it accepted upstream. I've tried writing an auth plugin for openid and it's not easy within the constraints of how the auth plugins work - to the point where I don't think it's worth the effort when persona auths against the same system and it's already included upstream.

I'll see if I can figure out how to add details about using fp.o email aliases to the login screen - the process is certainly unclear

I took a stab at making the process at least a bit more documented. Take a look at the following link (if you're not logged in, may need to reload for cookie refreshing):

https://phab.qadevel.stg.fedoraproject.org/auth/

I realize this still involves more complexity than it should but do you think it would at least add clarity to the process and remove some of the confusion?

That looks great, Tim. I'd put fasusername@fedoraproject.org in bold, it's the most important piece of information.

When creating a phabricator user account, please match the account name with your FAS username for less confusion on everyone elses' part

I thought we actually hardcoded Phab to use "fasusername" as their Phab login name. But if it's not trivial to do, the instructions certainly help. Thanks.

! In #437#6502, @kparal wrote:
That looks great, Tim. I'd put fasusername@fedoraproject.org in bold, it's the most important piece of information.

After much delay, I made that change

When creating a phabricator user account, please match the account name with your FAS username for less confusion on everyone elses' part

I thought we actually hardcoded Phab to use "fasusername" as their Phab login name. But if it's not trivial to do, the instructions certainly help. Thanks.

I'd like to but haven't found the time to write a plugin to actually do that. For now, it'll just have to wait unless someone else is interested in writing the plugin to make a more FAS specific login plugin

Since there have been no comments on this for a while, I assume that it is fixed enough and am closing the ticket. Having a better auth mechanism is covered in #552

If there's something I missed, please reopen the ticket