So, I opened a pull request for specifying a custom download URL when backing up to B2. This allows you to get free data downloads through Cloudflare. I’m using this now myself and it’s working great.
I figured I could try it with the WebUI by replacing the duplicacy binary and manually adding the keys in the right places to the preferences files. In practice though, it looks like the preferences files are generated as needed and the “keys” section that I add is overwritten every time.
As far as I can see there isn’t a place to add keys and values to the storage configuration anywhere in the WebUI. As the WebUI seems to mostly use the duplicacy binary for all tasks, I think it’d just be the configuration interface that is necessary to support the custom download URL.