You can create a VirtualNode in the target account using AWS Cloud Map as service discovery mechanism pointing to the existing ECS services - look at  and . When sharing the mesh across accounts using the AWS Resource Access Manager  this virtual node becomes available in the other account and can be accessed like a regular VirtualNode and will automatically route the requests to the services defined via Cloud Map in the target account.
You can check out https://github.com/aws/aws-app-mesh-examples/tree/master/walkthroughs/howto-cross-account for an example.
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