It is possible to use output Export and Fn::ImportValue to reference outputs of a stack from another one.
E.g. to make the DB name available as an environment variable in Lambda.
But once an output is "used" (via Fn::ImportValue) by another stack, it cannot be changed. So that destroys the value of using cross-stack references.
The chosen alternative was to fetch the stack output values (the real values) and inject them in serverless.yml (no references).