The connect-service-upstreams annotation takes an optional third argument to specify the data center in which to resolve the upstream service.
A pod in DC1 configured with an annotation of consul.hashicorp.com/connect-service-upstreams":"backend:1234:dc2" will always route connections received on port 1234 to a service named backend in dc2. Mesh gateways can be used to provide service-to-service connectivity between the two data centers.
You can also configure this explicit redirection, failover to DC2 by creating a service-resolver. See the Redirect and Failover stanzas for more info.
Example config entries which utilize these stanzas can be found in the service resolver docs, or this post on Implicit connection across datacenters - #4 by blake.