Well I think [face=SPIonic]pepurwme/nhj[/face] can be justified if we imply [face=SPIonic]xalkoliba/nou[/face], i.e. the genitive of [face=SPIonic]h( xalkoli/banoj[/face] "burnished brass". (Let's thank Koala who pointed to [face=SPIonic]h( ka/minoj[/face]). It seems possible because this word was just used in the dative and the participle applies obviously to such a material.
In my opinion it could be a genitive of material (Smyth 1323) : "His feet were like burnished brass, [as made] of [a one] like tested by fire in a furnace", hence Rainbow Missions' translation on Perseus : "as if it had been refined in a furnace", and the French translation from the "Bible de Jérusalem": "ses pieds pareils à de l'airain précieux que l'on aurait purifié au creuset".
[face=SPIonic]xalkoliba/nou pepurwme/nhj[/face] could be also interpreted as a genitive absolute, but it seems less convincing to me.
Technically speaking, [face=SPIonic]pepurwme/nhj[/face] is a lectio difficilior than [face=SPIonic]pepurwme/noi[/face] from a syntactical viewpoint.
Here the author shows something like the contrary to Daniel's clay feet colossus : Dn, 2, 31 "You, O king, saw, and, behold, a great image. This image, which was mighty, and whose brightness was excellent, stood before you; and the aspect of it was awesome.  As for this image, its head was of fine gold, its breast and its arms of silver, its belly and its thighs of brass,  its legs of iron, its feet part of iron, and part of clay. "
Then it seems important that the "testing by fire" should apply to the material, not to the feet themselves. because it would somewhat disrupt the scene's dramatic intensity : imagine "the one like a son of man" with his feet in a furnace ! Or the feet in a furnace while the "son of man" is waiting outside for the end of the testing !