Obnam could do with a plugin that just compresses data, without encrypting it. Encryption might be unwanted, or unnecessary, for example when backing to a USB drive that uses whole-disk encryption. However, saving space by compressing data with gzip or xz is still a reasonable thing to do.
The compression plugin would be similar to the existing encryption plugin, but be triggered by a different setting.
compress-with: program to compress files with; default to gzip, but can be any program
--liw