I was recently playing[0] with the ZSTD seekable format[1]: a *valid* ZSTD stream format (utilizing ZSTD skippable frames, that are ignored by de-compressor) w/ an additional index at the end of the file that allows for random access to the underlying compressed data based on uncompressed offsets. This combined w/ a Content-Defined-Chunking[2] and a cryptographic hash[3] in the index allows for a very efficient content-addressable storage. For example I've successfully applied it to a bazel-cache, which gave me between 10x and 100x wins on repository size w/ negligible CPU usage increase.
[0] https://github.com/SaveTheRbtz/zstd-seekable-format-go
[1] https://github.com/facebook/zstd/blob/dev/contrib/seekable_f...
[2] e.g FastCDC https://www.usenix.org/system/files/conference/atc16/atc16-p...
[3] https://github.com/facebook/zstd/pull/2737