A good trick I got when I was doing one of the TQAs around. Here is the details:
Situation:
Coexistence between Exchange 2003 and 2010 OWA and the proxying is working just fine internally. When try to 2003 user try to access the published URLs (using 2010 FBA), he’s getting another FBA page for Exchange 2003.
Solution:
Remove the Legacy publishing from the OWA publishing rules (paths and accepted domain names). Create a new dedicated publishing rule for legacy that has the legacy URLs and the domain names and point it to Exchange 2003 FE directly.
Works like a charm! Try it and feedback!
If you like the post, don’t be shy and hit the rating button up there 
Mohammed Saad
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