Yes, that would be a clean way I guess. I also thought about having two properties - one is the default-blueprint - and the other the instance.
The instance-property could be assigned in the construction-script by finding instances of the blueprint.
But I must admit, that I do not really see the point in disallowing editing instances in blueprints, since I would “left that to the implementator”.
This is mainly because I think the above workflow is more complicated than it could be… because “it is only one property…” ? :
Really nothing critical…