Like the idea, but instead of adding yet another currency I think you could simply use the lesser Tomestone as the currency for previous dungeons. No real need to add another and you shouldn't be able to buy the current dungeon gear (current patch dungeons).

And honestly this could remove people from DF, so that could be an issue.

So to combat that if you're seriously after a piece, I think the pieces should be expensive. Say 2,000 tomestones per piece. This way it helps to keep people in DF for that much longer.