Hay que modificar la ISO y arrancarlo con el MPHgameloader
De la pagina de MPH:
It works but you have to rebuild the iso with vcdromx and not umdgen.
Replace the prx's with decrypted ones that you decrypted with decryptPRX 1.0.2.
Open up boot.bin with a hex editor like hex workshop and find this 59A1 7915 560C D564 among one of the lines (in my case the full line is 00099C70 59A1 7915 560C D564 51AF 6C13 2D6B D5E2).
Replace the 16 digits mentioned earlier with 0000 0000 0000 0000 and save.
Recompile the iso with vcdromx and boot like gta.
This method was used for pacman, and thx to whom ever discovererd it.
Yo le he metido los prx del metal gear acid, hay que editar el boot.bin y meterle 16 ceros en esa linea y reconstruirlo con el vcdromx3.6
Comprobado por mi, funciona perfecto y se puede grabar la partida.