I had 4 Express segments (twice LAX-SNA return) included in my Star-rtw in sep-98 (the pre-PIP-rtw). LH-ZRH did (wrongly) allow this and told me, just weeks ago, that UA later charged them seperately (something like a fine/compensation) for this fault.
When I booked (for my children) their rtw for spring 2000 with UA-FRA they refused the Express-segments (and faxed me the rules confirming that they were right).
It seems that your (UA-Express-) chances are fair if you buy your rtw not with UA.