Hey there! You're right, there used to be a Sephiroth playable mod for 7th Heaven. The thing is, it wasn't quite a full playable character experience.
Here's the deal:
Original Mod: There was a Sephiroth mod for 7th Heaven that let you walk around certain areas as Sephiroth, but it wasn't a complete playable character with full functionality.
Not Currently Available: That specific mod isn't included in the official 7th Heaven package anymore.
However, there might be hope!
Alternative Options: Look into recent modding communities for Final Fantasy 7. There's a chance someone might have created a more complete Sephiroth playable mod. Try searching forums or websites dedicated to FF7 modding.