I'm using the new "get data from xml" step in 3.1

I need to tell it to get it's input file from a directory part of which is a parameter. I'm going round in circles as I can't seem to expand the mixed pathname

c:\mydata\%MYDIR%\*.xml into a full path name
getFiles doesn't want to expand the variable or take a field as directory and the getXMLData step wants either a fully qualified filename in a variable or a constant plus a regex

Is there any way round this?