I have used them and the trick is a small strap to the ring it can be larger once it is outside of the mouth. The small strap will allow you to have sub put her tongue through the ring and place the ring further back in her mouth allowing the lips to be enjoyed her tongue too.

The gag factor will go up and it will take her awhile to get used to it but it can be quite thrilling to see her like this and you can have her use her tongue to massage cock either in her mouth or just outside of it.