Sorry if I'm distracting from what you're talking about, but have you decided what's going to replace the adamantine yet? Because if not I think I may have an idea.
No. I did suggest this but it didn't catch on. What's your idea?
I was just thinking that rather than making it another relatively generic "this metal has really good material attributes" version of adamantine, it would be nice to have an incentive to breach hell for some other reason (particularly since, let's be honest, there are probably going to be several supermetals in this mod by the time you're done). One idea I was having was that rather than the boulders dropped being workable into a metal, instead you could put in a reaction to convert the boulders into artifacts. I don't know how possible that is, but it seems like it should be so long as there is a way to control the quality of a product from a reaction. This property would be sufficiently unique to encourage breaching hell, particularly if there were some way to control which artifact you got back. I know that I, for one, would be pretty tempted by the lure of indestructible doors, unjammable mechanisms, full suits of artifact armor etc. But, as I say it's just an idea, you're running the show and should take it where you want. If you do like it, I'll start trying to find out if it's even possible. I'm not sure, but I think some of the reactions in Masterwork do something similar?
EDIT: Of course, that being said, I do also like Leibowitz's idea of making it entirely undiggable, just to make hell a bragging-rights reward as well as providing some utility. And I agree, what on earth was Armok thinking when he gave his uber-metal a weakness to copper? The only problem is that, so far as I know, the methods you're thinking of only work from the bottom up without some rather funky glitching. If we made the adamantine undiggable, then we'd have to go through SMR in order to get at the stuff (not that that's impossible at the moment)