A mirror that re-compresses packages is a pretty obvious step from any discussion about package compression but it has the potential to break a couple of longstanding assumptions. Most importantly it means serving different bytes than the publisher sent. It could mean serving different bytes with the same filename even if the content underneath the compression is the same.
For example, a mirror might recompress the latest releases of the top 512 packages, serving a simple index that would include those recompressed packages on their per-package pages /simple/squished-package/, every other file of those top 512 packages (older releases) and every other package would link to or redirect to the ordinary index.
If your client supports the format, point it at this new index. Boom, less bandwidth. How broken would this be?