Apple hasn't been keen on console emulators apps so far, most probably to avoid any copyright drama. If developed officially however, there would be no reason for Apple to reject it and this might be happening sooner than you think.
Indeed, according to Gizmodo, Sega will releasing today a free official emulator named "Sega Genesis, Ultimate collection". It will include "Space Harrier II" and additional games will be available through in-app purchase. Here's the rumored lineup:
• Sonic the Hedgehog: $5.99 • Golden Axe: $4.99 • Ecco the Dolphin: $2.99 • Shining Force: $2.99
I just activated AppAdvice's digitronic app release scanner, so at the moment it goes live, you'll be the first one to know.
Gizmodo got apparently a little bit too enthusiastic about the whole thing and Sega would be only launching it in February... We'll keep you posted.