As of April 1 2015, SF II Sheng Long is in fact REAL!
Real for your own MUGEN purposes!

Here's how to fight SFII Sheng Long!
Download SFII character pack from: http://gcnmario.free.fr

Player must be either Ryu or Ken using sf2ww.def. sf2ww/ryu.def or sf2ww/ken.def will NOT work.
Must get at least 5 PERFECT matches against anyone. Do NOT lose any health at any time.
Draw the first round against M.Bison when you've reached him as the 5th match onward.
Do NOT use sf2ww/mbison.def for the CPU match. This will NOT unlock the fight against Sheng Long.

Arcade Notes for sf2ww.def:

A random character between slots 0 and 7 will be chosen for the 1st match.
Balrog always takes 2nd match.
Vega always takes 3rd match.
Sagat always takes 4th match.
M.Bison will always take the 5th match onward.