A bit tricky to determine since I'm not super familiar with the API structure here.
I'm assuming that a ConfigObject would wrap around an entire JSON value/environment variable. From that point onwards you need a visitor that traverses the wrapped value—it looks like you expose enough to make that possible, but I'd need to play around with it to find out.

From a quick look at the API is also not clear to me how it is going to square out with bindings to an existing struct via serde?

I'm assuming that a ConfigObject would wrap around an entire JSON value/environment variable.

Yes, a ConfigObject is essentially a complete configuration that is loaded from a source and is then one layer in the layered configuration.
From there you can retrieve a &dyn ConfigElement which is one element of your configuration. For example a boolean, a integer... or a list or a table... It could be a serde_json::Value, or a toml::Value... or something different.
And that can be used to traverse the whole tree of configuration settings from that one ConfigObject/Layer.

ConfigElement is implemented on serde_json::Value for example (https://github.com/matthiasbeyer/config-rs-ng/blob/master/src/element/json.rs). That would be the "backend" of the config-rs-ng crate so to speak.