The Camino Lebaniego is a two or three day walk, started most easily from the Norte at San Vicente de la Barquera. The Lebaniego ends at the monastery of Santo Toribio, where the Camino Vadiniense starts to take you down to Mansilla de las Mulas where it merges with the Francés. So I think it would be a hard detour to make from the Francés, unless you took a bus or train from somewhere on the Francés (like León) up to San Vicente.
The day walking into Potes on the Lebaniego is quite beautiful, but when I walked it there was a lot of harrowing road walking on a very busy road. I believe there have been some changes to reduce the amount of road walking. Since you commented about beauty, IMO the real beauty comes the two days afterwards on the Vadiniense. From Potes to Fuente Dé or Espinama, and then the next day on to Portilla de la Reina are just gorgeous walking days.
