If you want to secure your seat, you could just loop something under the mounting rails? I plan to do this with the next bike i get, as i'll be riding it much farther than the one i just laid to rest. I am not sure it's worth replacing screws in my instance, but i guess you could end up disuading potential thieves. There are other screws that you could use if you're willing to machine them yourself or having a shop machine them for you...

- Andy