Hmmm... okay, I'm able to work around the bug in the following while visible is set to true, but once I run the script with that set back to false, I get the same 'no window' response to a click on the dock icon.
Code:
tell application "OmniFocus"
tell the default document
set theWindow to make new document window with properties {visible:false, perspective name:"Inbox"}
close theWindow
end tell
end tell
It may be that the best option is a script that opens the window, does the export, then closes the window for the time being. Apologies!
(Attaching this info to the bug I filed, as well.)