Hi @sander!
I'm using your ListAppender inside a custom transformer and it's throwing python error. This is the message:
INCLUDE -- failed to evaluate Python script `def ParamFunc():
 return FME_MacroValues[FME_MacroValues['ListAppender_WORKSPACE_NAME'] + '_' + 'APPEND_VALUES']
value = ParamFunc()
macroName = FME_MacroValues['ListAppender_WORKSPACE_NAME'] + '_APPEND_VALUES_WWJD'
if value == None:
 return { macroName : u'' }
else:
 import six
 try:
  value = six.text_type(value)
 except UnicodeDecodeError:
  value = six.text_type(value, 'utf-8')
 return { macroName : value }
'
Program Terminating
Translation FAILED.
Traceback (most recent call last):
 File "<string>", line 5, in MF_Include_1546897172100
 File "<string>", line 4, in ParamFunc
KeyError: u'$(test_WORKSPACE_NAME)_ListAppender_10_APPEND_VALUES'
No issues when ListAppender is on the main canvas.