How can I make an Async Slave track all of the nodes in a Percona XtraDB Cluster?
You can get rid of the pager, but not if you expect >= 50% failure.

PXC (Galera), spread across 3 geographic locations, is an excellent HA configuration.

Optionally, one of them can be a "garbd" -- light weight, no data -- node only for providing a "quorum".

If any one server goes down, the other two declare that they have a quorum and continue running.

If the network goes down, and 2 servers are split from the 3rd, then the 2 declare quorum and the 1 gracefully fades away.

"Normally" you need to plan for a single point of failure.  That is what the above provides.

If you really need to survive (without a midnight page) two failures, then you need 5 (or more) nodes in the cluster -- and place them in 5 geographic locations.  (Else a tornado, earthquake, etc, could take out multiple nodes.)

I am unclear on what you mean by a "Slave tracking all nodes".  Perhaps you want to set up two 3-node clusters with async replication from one to the other?  That is possible, and it has some degree of failover.

Some people even put the 3 nodes in a single location at one end; the the other 3 at a single location at the other end.  This gives them most of the HA, but without the dreaded latency you may experience between the 3+ nodes in a WAN cluster.

Depending on where your clients are, and which node each client prefers to talk to, a WAN Cluster may be both faster and more reliable than any non-Cluster configuration.  (That is another discussion.)

As for "all but one node can go down" -- that is [probably] a theoretical impossibility, especially when you consider that "split brain" is one of the failure cases.  The Cluster depends on discovering which subset of more than 50% of the nodes are still talking to each other.