You would need mod approval to develop abilities past what's been already approved, yep. But so long as this semi-AS is temporary and limited, you're good to go.

(For something like this, you wouldn't need to use Mod Contact as well; however, do please wait for our response before going ahead with it ICly! Couldn't tell if you were going ahead with it in your Link post, but we'd appreciate it if you waited for our response first in the future.)