Here is a full list of Spell Mastery Abilities let loose in this saga. Some of them are experimental and never been used. We brainstormed them up a long ways back and decided to let them fly and see what works. There are a few that have special requirements, as you mentioned, but they are not as stringent as you imagine. According to RAW, anyone with the slightest Mercurian influence can take Mercurian Masteries. Reference Societates, the Flambeau section, for further details on this. Roberto is Mercurian, though not a cultist nor interested in any Mysteries. Virtues such as Flawless Magic, Mastered Spells, and (others I forget) are all that a character needs to qualify.