Would you be willing to own the abstraction for multiple backends? The code is currently only a bit hardcoded to S3/AWS, but I assume most of the "work" will be discussing how to abstract different transports, exponential backoff, resumable uploads, and so on.

Fwiw, the GCS client for go (import "cloud.google.com/go/storage") is very straightforward. Though as others have pointed out, it might be worthwhile to just try to use minio-go if you want to gain Ceph as well.

Disclosure: I work on Google Cloud (and if the way is paved, we will contribute here; seems like a great project)