Save this as a toolbar or menu script. Select a task, and it will open (if exists) or offer to create (if it doesn't) a folder in /Users/you/Projects for the Parent Project of the currently selected task.
Code:
set currentUser to do shell script "whoami"
set projectFolder to "/Users/" & currentUser & "/Projects/"
tell application "OmniFocus"
try
set selectedTask to value of selected tree 1 of content of window 1
on error
--nothing selected
beep
return
end try
if containing project of selectedTask is not missing value then
set pName to name of containing project of selectedTask
set projPath to projectFolder & pName
try
set ls to do shell script "ls " & quoted form of projPath
on error
display dialog "Folder does not exist, create it?" buttons {"Yes", "No"} default button 1
if button returned of result is "Yes" then
do shell script "mkdir -p " & quoted form of projPath
else
return
end if
end try
do shell script "open " & quoted form of projPath
end if
end tell