I think this is ment as a seiyuu joke...

'tis indeed.

Pardon me, but I don't get how this is supposed to be a seiyuu joke. Kallen's not wearing Yukiko's uniform from Persona 4, and to my knowledge none of these character's voice actors (with the exception of Koshimizu Ami) appear in each other's series.

Maybe this should just be in the "crossover" tag instead? The artist did start to draw Persona 4 doujinshi after spending a long time with Code Geass and the Euphemia/Suzaku pairing.

And on top of that, Rise was voiced by Rie Kugimiya. Did she work on Geass too?

I think Toradora is the focal point here...because Kugimiya was Taiga...but that's the only connection I can make there