I reported the procedure to add slow channels to the FB. I guess you already have done Step.1
Tragically, this is more tricksy than I would have thought. The channels we need are "cdsEpicsOutput"s in the model. They don't show up in Dataviewer (fast or slow channels) or the regular fast channel .ini file. Jamie and I don't remember where these channels live and how to get them saved to frames. I'm on top of it though.