Well, SEGA definitely prefers Miku over Sonic this year. Maybe that will change when it's a Japanese made game again, but Miku got much more stage time at TGS.
I'm still surprised SEGA West is releasing Miku games. Do you think SEGA Japan is pressuring them? Surely you know! you seem to know stuff like this.
Also the first Miku concerts were put together by SEGA themselves, used an upgraded Project Diva PSP model rather then the fancy CGI that we see today at the Miku concerts
http://www.youtube.com/watch?v=8dmJeOPQqYQheres a video. Miku even says SEGA at the beginning!
also if you didn't know the person who is the main Miku artist is also the person doing the art for the
"Hi☆sCool! Seha Girls” anime. so, SEGA has certainly a lot of ties with Miku, even if it's not directly their franchise.