The first issue you'll run into is installing SSRS on multiple nodes. In my dev environment, I installed SSRS on EACH NODE first because it places permissions in both master and msdb databases.
These need to exist and hence you cannot do a standard install and then copy or rather backup the ReportServer and ReportServerTempDB databases as you would a normal "add a database" to AlwaysOn.
Once you have completed the install on every node, then you can drop the databases on the replica sets and continue adding them into the AlwaysOn process.
Then you had to manually go back and assign the vip:80/Reports as your web interface on all your replicas. Of course, when you failover, this may hang as stated above. In my environment I had to turn off the agent of the primary node that failed over to the replica. I had to leave it off for 1-2 min and then the web interface came back.
Reporting Services Always On AG for more details.
Reporting Services Database on AG also for more details.
Manual actions or custom automation scripts need to be used to complete the failover and recovery. Until these actions are completed, some features of the report server may not work correctly after the Always On availability groups failover.
I think this should be a BIG WARNING. But, like always, test, test, test and test again. Then you wonder, why didn't I just install this on a single server?
As our team discussed, even IF we got this to work, we dont get any "automatic" advantages as either our team or the applications team would have to be on-call for every failover to make sure the script "runs" or the web interface is up. Although we can do this, it adds complexity to maintainability in the future. What happens when you want to do an upgrade in-place? You can't just only do the databases, but now you have to add in Reporting Services and all the complex work you initially did.
This may be fine, but perhaps you might be elsewhere and some poor Joe gets this task in 10 years.
So make it easy for yourself, your team, and the future Joe's who have to support this database and application: Install SSRS on its own server.
Just my 2 cents.
Andy