I think I would have to create a view for each scenario: i.e. one view for the case where the user wants the RS to be limited to a certain Artist, another when the user wants the RS limited to a certain Genre, etc.
Being a view is not generated, there is no problem in writing a complex view that would have some insane query plan if run without a where clause, and then querying for each small case needed, resulting in a very quick runtime.
If the groups are small (artist, genre, etc), just create one view to order them all complete with row number, a column to identify which group it is, and perhaps what ordering is being used. Then, querying it for a specific group, ordering, or limit is simple.
select Lucutus from Borg where grouping in ('artist', 'genre') and ordering = 'popularity' and row_number <= 5; 