Turns out that the source code for EECH got released, and a group of dedicated modders have used that to keep it thoroughly up to date -- and they are still at it!
So, if you're looking for a great helicopter sim with a dynamic campaign, your steps to happiness are:
1) Buy EECH from GOG (all the mods require a full install first)
2) Go to this thread in the GOG forums: http://www.gog.com/en/forum/enemy_engaged_series/modding_eech_quick_guide
... which links you to the mods and to the SimHQ forums where the modders hang out.
3) Install the mods.
4) Fun! :D