There was a similar issue with nether quartz and existing world gen, but as extreme hills biomes aren't all that common, this would be much harder to find emerald in new chunks, especially as a large amount of the world has been explored...
The nether quartz issue was resolved because I found a plugin that placed quartz ore in the nether, in already existing chunks. I wonder if this could be done again with emerald?!
Well,
GOOD NEWS! because I found
this plugin, which is a little harder to use as you need to be in the biome for it to work, but I'm imagining that admins could come and regen emerald in people's extreme hills biomes, right?
Luis
Just in case that link did not work, I will post it again here!
http://dev.bukkit.org/bukkit-mods/emerald-gen/