The scene from the manga in which St Germain and cagliostro are mentioned (vol 5) seems to take place in Jun's dreamworld where he is turned back into a little boy ; he finds himself in a park where a mysterious man sitting on a bench is giving a shadow puppet show.
The scenes from the puppet show portray St Germain walking around with a glowing stone, while courtiers and ladies watch and comment, then we see Cagliostro and then a dollmaker in his workshop.
In the end the mysterious man with the puppets reveals himself as the Laplace demon bunny...
(two images from that scene -- but it's the chinese translation)
I guess we'll have to wait till volume 5 is translated (in july) before we know exactly what is being said in this scene (unless someone has read the manga in Japanese and can translate it for us ^_^).
Anyway, hermetic, occult and "gnostic" writing of old are really interesting to read ; there are even some available online (for example here's an English translation of a 15th century treatise on "the art of drawing spirits into crystals" :
http://www.esotericarchives.com/tritheim/trchryst.htm )
(And yes, Umberto Eco's books are awesome...)