xml is only a general vehicle for purpose of tcl/scripts automation. It is no different in concept than a "information.txt" which some script can handle.
Therefore, constraints could also be represented in xml format, as long as the scripts associated in your methodology could correctly parse it.
Really, it is just like you want to ship something(here is your contraint content) from one city to another, you have many ways to do so. You can choose to drive there, fly there, or walk!