Lack of support for cross-cloud peers relations

Although Juju supports cross-cloud provides / requires relations, it does not support cross-cloud peers relations at the moment. An example of a use case would be database replication across different clouds. Let's assume I have 3 MaaS clouds which are geographically distributed and there is Juju controller running in each cloud:

$ juju list-controllers

Controller Model User Access Cloud/Region Models Machines HA Version
maas-site1* percona-site1 admin superuser maas-site1 3 1 none 2.3.7
maas-site2 percona-site2 admin superuser maas-site2 3 1 none 2.3.7
maas-site3 percona-site3 admin superuser maas-site3 3 1 none 2.3.7

As Juju supports cross-cloud relation, what I would expect to be doable would be to deploy a percona-cluster application with one unit in each cloud, so something like:

$ juju deploy percona-cluster
$ juju add-unit -m maas-site2:percona-site2 percona-cluster
$ juju add-unit -m maas-site3:percona-site3 percona-cluster

The above commands should automatically create offers on one side and consume them on the other side, an establish "cluster" relation between the units.