Right now, External nodes use the default "Node Details" page, but the custom properties resource isn't visible.
I don't see a reason why you would NOT want custom properties to appear for an external node. As it works today, if an external node doesn't have a value for a particular custom property, it simply won't show up.
We utilize the feature (external nodes) to monitor devices for APM even when we don't care about the hardware. But having those custom properties is important so we know whose device it is, etc.