The chances of bricking the Rpi with overclocking are rather low I'd wager, most likely you'd just reduce it's lifespan, so accidentially bricking the RPi with software would be very hard.
(Has anyone tried to disable the overcurrent protection, to overvolt and overclock such that it doesn't just crash but gets too hot? It might be hard toget this right and very device dependant.)
However if you really wanted to brick it your best bet might be patching the start.elf files to (re-)programm the OTP bits in the chip. Code for the modification of the OTP memory is already implemented in the startup code somewhere to correct wrong boardrevision ids some boards got produced with. So there is binary gpu code available that does a potentially hamfull thing if modified. I don't know how far the gpu instruction set reverse engeneering effort has come, but I can imagine that someone involved in that effort could dissasemble / reverse engeneer the otp programming sequence and modify it to write all '0' or all '1' (depending on the type of memory) to the OTP memory, which might well brick it.
But why would anybody want to? Everything interesting is on the sdcard anyway which you can backup before trying it on real hardware so destruction of the hardware serves no real purpose.
The bottomline is - although it might be theoretically possible for a determined person to brick the RPi by software the normal user should probably not worry about it. And since i fail to see any "use case" for wich the destruction of the hardware would make sense i doubt that anyone would bother trying to figure out how to do it in the first place.