My Tumblr post was not a sheer forgery for the sake of attention, and AkaDragon did not intend that either since she found it somewhere else, please don't say that...
The cameo appearances do not have "true" / official sources. Whether they really will appear or not, I've no idea.
Edit: Shoot. Here's the video where her information came from:
http://www.youtube.com/watch?v=05L_sLhxC3A
I sent him a message asking for the sources of the cameo appearances. And from thereon, question the person who gave him the info for his video ><; woohoo for quadruple verifications.
Double edit: There seems to be absolutely no source for Nobuo Uematsu composing the orchestral "Dear You", since the video creator just left a blank. But the new opening song was released today!
Triple edit: I think it's safe to say that right now, Duke Nukem isn't (and shouldn't be) going have a cameo appearance. I just did a Japanese keyword search to see if there were any sites at all that connected 「ひぐらし 煌」 and "Duke Nukem" in atleast a sentence and so far, absolutely none. The only possible reason I could think of DEEN ever including him is if they turned one of the episodes into a crackfic and had him somehow save the day by blowing up the entire Yamainu company. (Or something crazy. Who knows.)
Erika doesn't seem too much of an absurd cameo appearance, given that she's been connected to Rika a million times, plus with that Episode X crossover manga that R07 released, it wouldn't be surprising for him to pull off a similar yet smaller trick again. But if Erika appears, wouldn't Bernkastel be requested too, and if Bernkastel appears, Lambda appears, etc etc...
As for Marisa Kirisame's dress cameo appearance, it'll just look like another traditional maid outfit to me (they're both almost identical, anyways.)