So, I notice that the custom attributes do not appear in the field selection dropdown after you select the object. However, have you tried creating the expression manually using the custom attribute?
I was able to create the expression and validate the process. You will need to test this out and update the thread if this works. As to why it does not appear in the list of fields in the Object, I do not know. May need to log a case with CA Support.
Hope this helps!