It is quite simple really:
import Rhino
import Rhino.DocObjects as rdob
import scriptcontext as sc
cps = [ob for ob in sc.doc.Objects if ob.ObjectType == rdob.ObjectType.ClipPlane]
selob = [ob for ob in sc.doc.Objects if ob.IsSelected(False)>0]
Rhino.RhinoApp.RunScript("_SelNone", False)
if len(selob)==1 and len(cps)>0:
srfs = rdob.RhinoObject.GetFillSurfaces(selob[0], cps)
print(srfs)
for srf in srfs:
print(srf)
id = sc.doc.Objects.AddBrep(srf)
sob = sc.doc.Objects.FindId(id)
print(id, sob)
sob.Select(True, True)
sc.doc.Views.Redraw()
With file demo_getfillsurfaces.3dm (204.4 KB)
Select the torus before running the script. It adds a bunch of breps as the filling faces. Disable to clipping planes and hide the torus to see.