It works well enough. You can find binary providers that charge by download quantity and use that style of plan to upload for free. (And upload is usually free -- usenet providers want more content, after all.)

Name your backups something inconspicuous and have your client generate an .nzb file for later recovery (has the unique mail ids for each component mail in it).

It's a little risky in that maybe providers would delete your backups if they identify a pattern, or if you name them wrong some studio may issue a (bogus) DMCA takedown against them.

Tarsnap is cheap enough that I wouldn't bother with any of this unless you're already using a binary usenet account anyway.