Can anyone direct me to a site which delves into this grammar aspect more deeply? Any Latin grammar book which I have used only gives this a cursory treatment. Is the subjunctive passive usage rare in classical Latin?
I doubt that any grammar would treat the subjunctive passive as a special category, distinct from subjunctive active, except to list the forms. The syntactic rules governing use of the subjunctive passive are the same as those applicable to the subjunctive active. Is something specific puzzling you?