this should be possible - apparently IT already sets up some stratum 1s in this way. A word of caution though if you intend to put several CVMFS repositories in the same bucket: Read/write permissions are tied to the user identity associated with the bucket, and it might be trickier to configure access on the basis of subdirectories, if you don’t want that all the repositories can be read/written together.
@vavolkl Thanks for the information. Actually I’m not trying to setup multiple repositories in one bucket; I just have access to a single bucket on the S3 storage, and since I already use it for other purposes I needed to understand if I could fit also the CVMFS backend there.