There is currently no support for demoting the files to cold storage, but I don’t think it would require a big changed in terms of backup.
Yes, you can add --no-backend-verification --no-auto-compact to have a setup that does not require that you can read remote files. I am not familiar enough with the feature to know if it still returns the cold-storage items in the file listing. If it does list the files, a better setup is --remote-samples=0 --no-auto-compact which will still do verifications based on the filelist, but not attempt to download any files.
The problem is not making the backup, that part is easy enough. The problem is the restore. We did some work for Glacier, but it was a mess since you need to figure out which files need to be moved out, wait a few hours, and then access them, and then put them back.
I think supporting this feature would introduce a similar restore setup.