This has consistently been one of the most popular previous posts. I'm glad to say it's no longer relevant.
Most recently, with Forsaken Masters patch, Zana offers the chance to run maps as though you were in a previous league - including legacy drops from that league. Recently confirmed, though I've suspected something was up from day one of the patch when Shavronne's Revelation was updated in patch notes.
And hell, I'm glad they did something about it. I'd rather see my "investment" values dropping than not, because of what it represents. I don't know that people will consistently run anything but Onslaught and in 6-groups, considering Nemesis costs 2ex, but prices are easy to tweak, and even if they aren't tweaked, when headhunter prices go high enough people will start trying.
As usual, GGG has come up with a better solution than I could ever have thought of. Major props to them, and this is why I'm a player and not a dev.