CRM 2015 Impersonating as SYSTEM user in a custom workflow activity or custom action

I've gone through the following two forum threads and I learned that Workflows has special conditions and is designed to ignore the GUID you pass to the CreateOrganizationService.

https://social.microsoft.com/Forums/en-US/a78cf15b-835c-486c-a3f0-457825f47332/crm-2011-impersonating-a-user-in-a-custom-workflowdialog-activity

http://stackoverflow.com/questions/15982883/how-do-i-run-a-crm-2011-custom-workflow-activity-as-a-privileged-user

But currently, I am developing one custom action which requires SYSTEM user privilege to read some config data. And the problem is that impersonation by passing null to CreateOrganizationService (run as SYSTEM) works if the custom workflow assembly is registered in Sandbox isolation mode. But it's not working if the isolation mode is changed to None.

Is there any particular reason (or an article about that I can read) why the behaviour is different in different isolation mode?

Is there anyway to make it work on isolation mode "None"? (since I've got some actions in the code which cannot be run in Sandbox isolation mode)