I am not familiar with webdav, and I'm no bazzite expert (although it's my daily driver), but I would guess that distrobox could be a way to do this.
Either install a distrobox of arch or mint or Debian or whatever, and do the install there, or check out what distrobox has to offer for mounting WebDAV storage. I found it has options for implementing local AI that I would never have dreamed of, so who knows what else it can do?
If you don’t get a solution here I would recommend asking on bazzites discord or forum. There are more helpful people with more insights into immutable there. If it can‘t be solved without layering they might even be open to having it included in the image by default.
thanks for the suggestion. how can I install rclone on Bazzite? A quick searching didn't bring anything clear. Are you referring to using rclone via Distrobox or something?
I'm not familiar with Bazzite but I do have a SteamDeck with SteamOS and I do have things installed on it. There is for it 2 ways :
keep binaries in home, e.g. ~/bin/rclone because that does not get deleted on updates
allowing SteamOS to not be read-only and add a package. Here it seems to be via ujust but I don't see rclone in there so I'd go down their list https://docs.bazzite.gg/Installing_and_Managing_Software/ and it seems brew install rclone would work.