The basic idea is that we can just open up the plist in whatever application happens to be appropriate; probably, but not certainly, that is the Property List Editor. There are a few ways to accomplish that. I decided, more or less arbitrarily, to use System Events to open the file.
However, we do need to deal with the case when there is no environment property list file. What I did was to put a string representing an empty plist onto the clipboard, and then run a shell script that uses
pbpasteto dump that string to an appropriately named file in the preferences folder. This was actually the second approach I took. The first I tried was to just write the file directly using AppleScript, but this associated the resulting file with TextEdit, an undesirable choice for working with a plist. I didn't explore the reason, but assume that AppleScript defines (inappropriate) type and creator codes for the file.
One other odd thing I encountered was that SEE doesn't show an ellipsis "…" correctly in the menu. I just used three dots. Whatever.
Here's the AppleScript: