labeling - ArcGis label last vertex Z value

Is it possible to label a feature in ArcMap using geometry values?

In this instance I want to label features (polylines) with the Live Z value of the last vertex obtained from the Shape.

However when I try something simple like the XMin of the extent of the shape:

It returns an error:

Is it possible to use the features' geometry to formulate a label? I do not want to calculate values from the geometry into a field and then label that field unless I absolutely have to:

Ideally the label should be live, changing as the feature is modified.

Based on the answer by Felix:

def FindLabel ( [OBJECTID]  ):
  mxd = arcpy.mapping.MapDocument("CURRENT")
  layers=arcpy.mapping.ListLayers(mxd, "WatrcrsL")
  lr=layers[0]
  with arcpy.da.SearchCursor(lr, 'Shape@','OBJECTID = '+str( [OBJECTID] )) as cursor:
      for row in cursor:
         a=row[0].lastPoint.Z
  return a

Will label the features with the last points' Z value live!

Answer

Example for point layer

def FindLabel ( [FID] ):
  mxd = arcpy.mapping.MapDocument("CURRENT")
  layers=arcpy.mapping.ListLayers(mxd, "ac_pipes_aspoints")
  lr=layers[0]
  with arcpy.da.SearchCursor(lr, 'Shape@XY',r'"FID"='+str( [FID] )) as cursor:
      for row in cursor:
         a=row[0]

  return a[0]