How to put lines exactly onto a surface?

 From:  Ditto
6171.16 In reply to 6171.10 
> [Drawing Aids]
> DisableOnSrfInOrtho=y
> Switch that to =n and then you should get the snap behavior that you want.

That would be nice to be able to toggle this behaviour with a shortcut. Can I temporarily change program options during runtime? By 'temporarily' I mean, options are changed, but not saved.