Upon looking at your client implementation and API documentation, it appears that Artifact.Source.URI may be something that’s fetched over an API at hosted.mender.io and that there is no client-side control to the artifact URI returned.
I suppose I could try to patch the client to rewrite the returned URI on the fly in order to test out alternate paths in the hope that AWS S3 will route them to the same bucket. However this example URI from a tutorial implementing a custom Mender client, it looks like an AWS pre-signed URL, and it specifies that the host header is part of the AWS4 signature (see X-Amz-SignedHeaders=host in the parameters). So that approach is likely to result in AWS rejecting the request if I modify the host string.
"uri": "https://s3.amazonaws.com/hosted-mender-artifacts/5b48937d7e71f600014ab529/af0ccfb3-c410-40f8-9b30-649e5dd7878c?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Content-Sha256=UNSIGNED-PAYLOAD&X-Amz-Credential=AKIAQWI25QR6NDTJ7DLD%2F20191213%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20191213T204753Z&X-Amz-Expires=86400&X-Amz-SignedHeaders=host&response-content-type=application%2Fvnd.mender-artifact&X-Amz-Signature=3923b3238890ea98e51c6943805900570f306d1aaa858eedfd6b3ef1417dfd1b"