Hehe, comic had me chuckle.
And fragility isnt even the only problem there, the strength of an axe is that allmost all the weight is in the head, allowing for great momentum with a weapon as light as possible. Battle axes were kept as light as possible, with heads much thinner then chopping axes.
I think the [DEEP] tag was in .28 and it should be still working now. I just cant find any info on it.
The deep tag works, but it will make the material inaccesable for all entities. It also has the sideeffect of spawning the stuff in demon forts.
As for the whole discussion about iridium-would-make-good-sword, it wouldnt. Im pretty certain medieval blacksmiths had little trouble making their weapon heavy enough, this went even for blunt weapons. In fact, they went to great lengths to keep their weapon light, flexible yet sharp with things like fullering, pattern welding and all sorts of heat treatement (chating rituals and all. Those actualy did have an influence on the hardness of a weapon due to length of the heat treatement chants). Iridium in pure form fails at all those aspects (possible the stuff can be given a good edge, but it would chip very quickly).
The only use I can think of for the stuff is to use it as the head of hammers or the points of flanged maces. The greater hardness could give a minor increase in how much the weapons energy is focused on a single point. Oh and iridium is also incredibly resistant to wear. Coating steel tools and weapon in iridium could make them very durable, too bad they dont wear at all in DF.
As for modding this all in... composite material weapons arnt in the game yet, but you could at least make some fictive iridium allow. The idea of ultra-rare metals is neat, and could excuse some magical properties.