Apologies if this has been discussed before, but I think there are some relatively easy ways this could be made to work with a lower overhead and very little cost - and I have not been able to see these discussed elsewhere, so please feel free to ignore if they have been and I missed it.
Looking at the script there are two main phases to it:
This could be replaced with a simple DNS TXT record that contains the current feed version. Using the current greenbone.net TXT record as an example, the code would simply be:
host -t txt greenbone.net | grep 'MS=' | sed -e 's/^.*MS=//' -e 's/"$//'
This does the rsync directly to the existing directory. This could be replaced with a similar technique as used by Debian/etc using a signed file with checksums. The client could then do checksums, and just pull the modified files.
By using URLs that include the version, the fact that the DNS record and filestore are actually from different sources should not cause any trouble - and by using HTTPS as the transport method a CDN could be used.
This would then take the load off the servers for the subscribers to use the existing method, since that provides better support for authentication/etc.
If this would be of interest, I can write up the logic (or even provide such a script).