A node selection dialog can have a node predicate, which will be propagated to the data storage inspectors of the node selection dialog. Node predicates only check the validity of single nodes.
In order to check the validity of a whole selection (a group of nodes), a lambda function can be used to define a std::function. This function accepts a list of nodes (the selection) and is applied to each node of the list, such that any condition, which is defined by the lambda function, has to be fulfilled.
The custom lambda / std::function will be defined outside and given to the node selection dialog.