It would have to be thin and strong. And most keyways have broach grooves that cross each other, hence a thin strip would have to bend to fit into the lock.
Right, but it needn't be full height - it could be just deep enough to slide along the first convienient ledge below the pins (as long as eough material was left to hold the ramps together.) This is what would make a strip more general - it only has to deal with half of what makes one keyway different from another. When the distance down to the first ledge varies a little between keyways it would just mean that the strip has to lean to fit in.
...
I just realzed that even if a type of plastic can stand up to the bumping and bending, it has to be able to apply enough torque to turn the plug. This might make some type of metal a better material in most cases, especially if the strip is not very deep (vertically).