You would only apply the compression on the internally stored file and then decompress when retrieving it for the customer again. That way all the hashes and original structure of the user are retained.
This puts a serious limitation on your compression. You would only be able to re-do the entropy coding part of DEFLATE, which is actually pretty good.
You would still need to store the original Huffman tables for each block, so you can reconstruct the entropy coding exactly.
I doubt this would even gain you a single percentage.