@ilja So far, the easiest workaround (that was surprisingly easy for us) was to use an Object Storage service that puts everything in IPFS, but serves it back with an S3-compatible API.
After a lot of bumping around and figuring out the pieces, it ended up working better than expected! In practice, though, it just works like regular Object Storage. A next step might be to try to make Mastodon serve back the CID and render it directly, instead of proxying the S3 endpoint.