How can I reference a YAML “setting” from elsewhere in the same YAML file?

I don’t think it is possible. You can reuse “node” but not part of it.

bill-to: &id001
    given  : Chris
    family : Dumars
ship-to: *id001

This is perfectly valid YAML and fields given and family are reused in ship-to block. You can reuse a scalar node the same way but there’s no way you can change what’s inside and add that last part of a path to it from inside YAML.

If repetition bother you that much I suggest to make your application aware of root property and add it to every path that looks relative not absolute.