West End - Good
West End as Joseph - Bad... like really bad.
This is a ticket to nowhere; witness all the other ex-Josephs now appearing on bad celebrity-based reality shows. I don't want to dis the play, but this would not be a good move for Zac.
In fact, the only musical I'd like to see Zac in at this point would be Damn Yankees. To see him do Near to You (if it remains as the 1994 revival was) would melt my heart. Plus Shoeless Joe is right up his sports-loving, age-defying role alley.