Hizagkuur (post-errata, +3,000gp to the cost of the weapon) is a metal that is magickally worked, formed, and given it's properties.
Fever Iron is another ... +1,500gp, does +1[fire]. It is in a state of constant near-molten-ness. It's properties arise form having been exposed, in molten form, to powerful magicks for extended priods of time, such that it never truly -cools- into a non-molten form. Through magicks, it is turned into a workable metal that will hold it's form, and not burn people who handle it casually.
It retains it's properties in antimagic fields, too. Despite those properties being BORN of magick.
Now, the thing is: weapons with a (IMO) permanent enhancement bonus, get an equal increase to hardness and hitpoints.
If it were JUST to hardness, I'd agree, it was a result of the ongoing enchantment. But ... hitpoints ... ?
That sounds like a structural change, a byproduct of the intense magical energies introduced to it during it's manufacture -- and an effect which is, itself, nonmagical in nature.
I would let GMW handle sunders from and to magicl weapons, no problem. But since it's Enhancement is nonpermanent, I would not apply structural benefits (hardness and HP).
It avoids the "uh oh, antimagic field, my badly notched sword goes *pop*" problem entirely, for starters.
