Sorry no pictures. Best way to use custom face rig in rigify is to add one before generating rig. If done right you will get metarig with your custom face bones which can be regenerated at any time. You will need delete all face bones on metarig and add your face bones on metarig face layer - you can just join your armature to metarig. If your face rig has constraints, you must set up each custom bone rig type - either basic.super_copy or basic.raw_copy and check Relink constraints option. Parent your face master bone to metarig head bone and basicly you’re done. Also if you choose basic.raw_copy type for bone it wont have deform flag checked. To make such bone to be deformable you must add “DEF-” prefics to it’s name. That’s way your bone will be deformable and will be placed to rigify deformation bone layer.
If you want siply add face rig to already generated rig you should parent your face rig master bone (parent for face rig) to “ORG-head” bone - it’s placed on last layer I think.
If you get troubles just share your file I will look what’s wrong there.
Good luck!