This question came up in the Online 10-day Retreat Nov 3, 2020, guided by Delson Armstrong. Day 2 was part of a daily 30 -minute discussion on the suttas, the Dhamma talk and reflections.
Slightly edited to improve readability
To the extent to when you’re balancing the Factors. When you require a little bit of Joy, you can bring in the internal smile to bring up the Joy.
But in terms of a physical smile, at that point I would say, when you’re in Nothingness, Neither-perception-nor-non-perception especially, you’re not so much in contact with the physical. Actually, pretty much not. It’s mostly mental, like 95% mental.
In that process, when you are dealing with the Factors of Awakening, and when you require any kind of Joy to be put in there, it can help to maybe use the smile, as it can help to use a smile as an anchor, as a carrier for that Joy. It will help to that extent.