Could very well be standard hardware, especially considering rust. Getting it out might be difficult, but there are plenty of options. In most cases where I have a broken studs in aluminum, I will TIG weld a nut to the stud. MIG works nearly as well, but makes more splatter. Let it cool a few minutes, then turn back and forth till it loosens up. Sometimes you have to repeat if the nut turns off. There really is no worse case here, if ya really bugger up the hole, drill it out and use larger hardware, or weld it shut and re tap. Just a thought, a toilet seat hardware kit(brass and plastic) would likely never be an issue. If you really get stuck, I could help, only cost materials and mailing.
