With this mod we are now able to play singleplayer using the online models. Use a trainer to change to one of the online player models(male or female), customize the model using the trainer, now using my mod press B to bring up the menu and select which character you would like the game to think you are, close the menu by pressing B and bring up the interaction menu and it should display the character's name(Michael,Franklin,Trevor). You are now free to play singleplayer using the online model. I have included the source to this mod if you would like to change the hotkey or expand upon what I have created.

Requires NativeUI and ScripthookVDotNet

Special Thanks to:
kr1ck on GTAForums for the discovery
GhostMode1 at mpgh.net for the tutorial on how to find pointers
Matej Tomcík on Youtube for showing how to edit memory in C#