Simply hosting a preseed file on github.com fails, for example fails.
This was an issue the debian-installer in ubuntu as well but they seemed to have added https support. Couldn't it be added to the original installer?
https://bugs.launchpad.net/ubuntu/+source/debian-installer/+bug/833994 * Add HTTPS support to fetch-url, which will only work if d-i has been built with GNU wget; debian-installer/allow_unauthenticated_ssl implies the --no-check-certificate option (LP: #833994).