I think 星君 might be a reference to one episode of "Chargeman! Ken" which more or less became a meme on NicoNico Douga. If that's the case then the pronunciation would be "Hoshi-kun", and it is also the name of a character also refereed to as "The Mysterious Handsome Boy" (which might explain the usage of it here?). And as with most things viral on Nico Nico, it's been crossed with Touhou in many different ways...
I've seen it used as a way of referring to Shou, but only really on NicoNico, but I'm not sure if this is where the nickname first originated.
JesusK said: To make a touhou comic, you need a pun and an explosion!
The wise teachings of Bomber Grape.
BAMSpin SpinPOPSHWFYour Excellency, I've come to report.GYULPWAAH!How's about you and I have dinner tonight?Heya, Nazrin, how ya doin'?Heh heh...you're such a cutie. Do I look like I care about social status?B-but Your Excellency! A mere rank-and-file soldier like me getting such an invitation is...KA-TASSSHShou Boy"Shou-kun" refers to Shou as if she were a young man. I went for a pun on "Oh boy". I welcome better ideas...