Make Exit Save just outside entrance to, let's say Concorde Civic Access, and copy the created file somewhere. Load it up and enter the area.

NOW THE IMPORTANT PART

As soon as the area loads(still a black screen but the game puts you in the area), but just before you see the area proper, make another Exit Save and copy it too, doesn't matter where you put it, as long as you know it's the area pre-load save. Go to the roaches and see if a legendary spawns. If there's none, load the one before entering the CA and try again. If a legendary appears, standard procedure for scumming commences, except you copy back to the Saves folder the area pre-load save till you get the prefix you want.

I'll post pics when not lazy. Instigating Combat Knife, here I come.

Edit: Just a PS that this will require more patience compared to non-Survival scumming.