This seems to me like an oversight, the C++ class is blueprinted into a blueprint class, just like how we can override the CDO’s CreateDefaultSubobject in C++, we should be able to override the Default Subobjects in Blueprint… After all, that blueprint class has to be read from disk anyways…
I guess this could be done via the ConstructionScript in blueprint if the UPROPERTY() has BlueprintReadWrite on it.