This is a question/request for enhancement. I found out (a year after the original commit upstream) that phabricator was enhanced to support a more generalized export feature However, when this was implemented the older extensions that allowed for the export for custom fields took a hit as custom select fields are not yet supported in this implementation.
Similar to the previous extensions I tried to build a custom extension but the way the new code has been implemented I don’t have a handle to options of the select field unless I pass a reference to the CustomSelectField object itself. To do this I will need to implement the following change
The ExportField file can be placed in extensions and I was able to test that on an image of phabricator. However, I am not sure if PhabricatorStandardCustomFieldSelect.php can be extended.
I need some guidance on either adding the StandardCustomField to extensions. If that is not possible then I would request for the change to be implemented upstream. I am open to creating a differential if the discourse topic is promoted to a phabricator task.