Ah yes, gmailfs[1] in 2004 used Linux FUSE (Filesystem in USEerspace) to translate local directories to your (at that time, rapidly and absurdly growing) GMAIL quota.

FUSE let Linux do all kinds of interesting things. Forgot about this one, thanks!

[1] DSLreports discussion from 2004 https://www.dslreports.com/forum/r11192502-GmailFS