Just to update, I have had a chance to play around with this a bit. I setup a 2-node cluster and installed the 4.2 branch. After setting things up more or less how we have them on our current system and making sure all was working as expected, I ran through the steps of upgrading from 4.2 to 4.4 as outlined here
I did the same testing I did earlier and everything seems to be working just fine. FusionPBX is indicating it is running 4.4.1. There are no errors in the Log Viewer. The ony thing was in Step 6 in the upgrade steps it said there would be blank duplicate entries in the "Email" section under "Advanced -> Default Settings" but I found none; just the settings I had previously in there.
I did the version upgrade steps on both PBXes.
So, to confirm, installing a cluster using the scripts (but forcing it to be v4.2) then following the steps to upgrade from version 4.2 to 4.4 *does* appear to work.
I haven't testing doing a straight-up install of 4.4 by running the commands in the scripts manually (so I can figure out what needs to be adjusted/fixed), but I *suspect* it is the part that installs the database, and making sure it is PgSQL 9.4 that gets installed and not an updated version. I am not sure when I will get a chance to do that (aiming for soon), but I just wanted to report back here on the results on my 4.2->4.4 cluster upgrade.
I hope this helps someone.
Oh, one other thing: after I finished the upgrade, I did reboot both my PBXes, and I ended up having to actually reboot my phone for it to re-register (I waited about 2 mins but it never seemed to retry). I probbaly didn't have to reboot the PBXes, but I usually like to do that after an upgrade anyway. I probably *should* have rebooted my primary PBX first, then my secondary when the first came back up; worst-case, the phone would have registered on PBX2 when PBX1 went down, then re-registered back on PBX1 when PBX2 went down. Lessons learned