It's still throwing the "File is read-only" error on my face every time I try to edit a code with a Story Editor. However, I have found a work-a-round that works in my case. Create the file manually:
In location:
\Divinity - Original Sin\Data\Mods\<YourMod>\Story\RawFiles\Goals\
Add a new text file with a name you want and with this content:
Version 1
SubGoalCombiner SGC_AND
INITSECTION
KBSECTION
EXITSECTION
ENDEXITSECTION
You can then move it under "__Start" section in the Story Editor and begin editing it.