That motherboard will warp very badly over time...do you really need SUCH a massive heat sink? If this is for a HTPC...just undervolt the i5 as much as possible. Also, does it have to be COMPLETELY quiet? Can't you get a really silent fan, and therefore a much lighter heat sink?
Seeing that ATX boards can support coolers like those mentioned it should be possible to mount those on a mini ITX board too - and I have an idea how to mount the heatsink to the mobo, but with the mobotray supporting the weight. From what I have seen, the weight of those tower-heatsinks is ranging from 650 to 870g - somehow thereÂ´s not much difference regarding the size.
And neither undervolting nor mounting a fan is an option (atm).
Guess IÂ´m sounding a little bit stubborn here so...
CouldnÂ´t find any details on how much heat those heatsink can handle in a passive setup.