I think the only way to mod EFLC *Right Now* is by having it on your HDD (whether it be a back-up or the Downloadable Version from the PS Store).
From what I can see, the needed file (script_network.img) is located on the Disc itself.. so there would be no way to mod it without having it all on your HardDrive.

I recommend you buy BoGT and TLaD off the store and just mod those. It's much easier and you can play it all on one disc.