Well this seems to be an old news but I just discovered it recently...
Two of the ending themes from Sayonara! Zetsubou Sensei(さよなら 絶望先生), both penned by Hashimoto Yukari, featured instrumental artists who used to cooperate with SR. It is rare to bump into "genuine" pop musicians in anime-song fields.
Season1 ED1 "Zessei Bijin" (絶世美人)
http://www.youtube.com/watch?v=jDHqR7iYm8E
Gt. Nagoshi Yukio (Tsumi to Batsu [other than Benji], Yokyou)
Ba. Watanabe Hitoshi (Meisai, Ishiki)
Season2 ED1 “恋路ロマネスク"
http://www.youtube.com/watch?v=I8PpySmFJcw
Gt. Nishikawa Susumu (Various SS-era songs, Spring Shower '99)