it work for me with remplace t5mp_ps3f.self
1: Get single player working
2: Copy the EBOOT.BIN at GAMEZ/BLES01031/USRDIR and rename it t5_ps3f.self.
3:Put file t5mp_ps3f.self
4: Remove the t5mp_ps3f.self from HDD/GAME/BLES01031/USRDIR
So at the end it should look like:
GAMEZ/BLES01031/PS3_GAME/USRDIR has EBOOT.BIN, t5_ps3f.self (copy of EBOOT.BIN renamed) and t5mp_ps3f.self
/DEV_HDD0/GAME/BLES01031/USRDIR has EBOOT.BIN only