Heheh, I find I spend more time editing the files than actually editing missions in game. 
A lot of that is editing the critters, though, and because I'm trying to avoid the bug that makes the face sliders revert back to zero. And editing height so it matches my existing characters.
I'm fairly certain that there is such a project here, though, I just haven't had a chance to look for a link to it yet.
Leandro made one. I haven't used it personally yet, but he did great work with the Splasher, so this should be good too.
Anyone more ambitious than me put together an offline mission planner yet? I realized I spend most of my time typing dialogue and exposition, sometimes over and over again, and it would be great to have a resource to work on it while out of the game, so I can just cut/paste over what I decide on.
Chekmate, Lvl 50 AR/Dev blaster
BlueEyed Murder, lvl 50 MA/Regen scrapper
Hard Feelings, lvl 50 energy/willpower brute
"The Bell" - stop a mysterious group with ties to a WW2-era mythic German superweapon. arc id 76773