The way I interpreted the question, it was all about whether or not computed column definitions can contain references to columns of other tables. The answer, according to Books On Line is NO.

Yes, there is a workaround, but it's just that - a workaround. It's not really a valid answer to this question because the computed column definition referenced a UDF - not an external column.

I have an issue when the whole purpose of the question is to trick the reader. A rephrasing of the question could have avoided the second-guessing of intentions.