Your chances of an upgrade on your upcoming flight -> 0.
ZED non-revs are only eligible for upgrades if their airlines' reciprocal agreement allows you to list for first/business class. Generally this would only occur if the two airlines enjoy a very close relationship e.g. United and Lufthansa or Northwest and Delta. WN employees don't have this privilege with UA (why would UA upgrade WN employees if UA employees can't be upgraded on WN?). When you were on a Buddy Pass in the past you were eligible for and listed in first, which is why you got the big seat.
I've *never* been upgraded to first or business on a ZED ticket unless I was listed for that cabin. And that would be, of course, at the bottom of the upgrade priority list. Although I have been given an occasional E+ seat on UA, which is probably the most you can hope for.
The only times I've been op-upped have been when I was travelling on regular revenue tickets. Which is the way it should be.
When are you hoping to travel? ZED TATL in the summer/early autumn is very ambitious, particularly for someone relatively new to the game (guessing from the questions you've posted). I assume you've got a few backup plans to get home?
Good luck!