MultiMC/MultiMC5 — pull request #28 by 02JanDal (2013-12-30 12:03:42+00:00)
Title: Button for opening an editor for the custom.json file

There is also a setting for overriding the system default editor

https://www.pivotaltracker.com/story/show/63044446
https://www.pivotaltracker.com/story/show/63044448