It doesn't make a lot of sense to enable it when Foreman is used as an unattended service(and puppetca), I think that it should be enabled only if you disable the unattended part.

How would you define the default node? what would be the class? would you select it from the list of classes, or would it be hard coded somewhere (e.g. in the settings.yaml file)?

In general, I see it as a bad practice having "unmanaged nodes" and the main goal with Foreman is to automate the whole process in a way that your nodes are always manageable.

The reason for having a default node is to get some basic configuration done while the system is waiting for a full config through foreman.

I have found a workaround which is usable:
just define the default node in the site.pp file