There is some opened questions - the standard (and some other databases) requires entering XPath expression as string literal.
I am thinking so it is too strong not necessary limit - (it enforces dynamic query in more cases), so I allowed the expressions there.
Another questions is when these expressions should be evaluated. There are two possibilities - once per query, once per input row. I selected "once per input row mode" - it is simpler to implement it, and it is consistent with other "similar" generators - see the behave and related discussion to "array_to_string" and evaluation of separator argument. The switch to "once per query" should not be hard - but it can be strange for users, because some his volatile expression should be stable.