Create a custom API accessible on phone where custom tags can be registered and data supplied by other apps for display on the phone.
Possible uses, continuous blood glucose readings could be available on the watch. (Example nightscout/xdrip)
Would be the responsibility of the supporting app to register the data and supply the updates to facer.
A simple sqllite db, or perhaps a simple registered directory with a file per custom tag, where the file simply contains the value in whatever the required format for facer would be, flat or json or whatever.
I would assume the facer app only sends data for the desired compiled tags currently, so syncs would only need to occur when the value changes for a compiled face that uses that tag.
Just some thoughts, would be willing to do the development for free if interested.