Is there a simple probe that just traces the lines in one routine? (trace.ual seems a bit of overkill)

Yes.  Use the following probe:

probe thread
{
  // Replace your routine name here:
  probe "MyPackage.MyRoutine"
  {
    on_line (all)
    {
      log ("MyPackage.MyRoutine line: ",
           ap_StringValue (ap_LineIdToNumber
                          (ap_CurrentLineId)));
    }
  }
}

When formatted, it will give output similar to this:

  MyPackage.MyRoutine line: 120
 MyPackage.MyRoutine line: 122