If you want to get or set the properties of a custom field, you cannot use $this-><fieldname>
I hit this issue when trying to add custom field variables to the Row_Rendered function, where the custom fields were at best being ignored, at worst giving me undefined property errors.
I couldn't find anything in documentation about this either.
After digging around the php code for my list page, it turns out that Custom Fields are differentiated with an underscore in front of the field name
In your Row_Rendered scripts you need to use $this->_<fieldname>-><Value or attribute>
so instead of (in the case of a standard field)
$this->MyFieldName->ViewValue ="Some text";
you need to use
$this->_MyFieldName->ViewValue ="Some text";