Many ways to skin a cat. While this use-case seems valid, It looks like something that would work better for some kind of nuance-flow where “customer” sees only nuance side and internal discussion happens on ticket. If “private” comments - that is comments with different policy than ticket itself do happen, then this would allow full private conversation.
While it is impossible in Phabricator as of now, one can make-do with available tools, for example: use private paste/wiki for internal comments and link to ticket. Person not able to see link, won’t see it (just “restricted object”), while internal discussion could happen in wiki/paste page.