Cel-shading - Best Method For Outlines?

I would leave the Line command well alone as it's very slow, relatively speaking.

However, if you're determined to use real lines, you could try using 3D lines. Here's a function for 3D lines:

;
; Adds a 3D line to the specified mesh.
; Note: 3D lines are only properly visible when rendered in wireframe mode!
; 
; Params:
; mesh     - Mesh to add 3D line to. If 0, a new mesh is created.
; x0,y0,z0 - Start point of line.
; x1,y2,z1 - End point of line.
; r,g,b    - Line colour.
;
; Returns:
; Handle of mesh the 3D line was added to.
;
Function create_3D_line(mesh,x0#,y0#,z0#,x1#,y1#,z1#,r%=255,g%=255,b%=255) 

	If mesh = 0 
		mesh = CreateMesh() 
		surf = CreateSurface(mesh) 
		EntityFX mesh,1+2+16
	Else 
		last_surf = CountSurfaces(mesh)
		surf = GetSurface(mesh,last_surf)
		If CountVertices(surf) > 30000 Then surf = CreateSurface(mesh)
	End If 

	v0 = AddVertex(surf,x0,y0,z0) 
	v1 = AddVertex(surf,x1,y1,z1)  
	v2 = AddVertex(surf,x0,y0,z0)  
	AddTriangle surf,v0,v1,v2
	
	VertexColor surf,v0,r,g,b
	VertexColor surf,v1,r,g,b
	VertexColor surf,v2,r,g,b

	Return mesh 

End Function

Using this method you have to do 2 renders: one normal render then one render in WireFrame mode to display the 3D line mesh. I've used this technique in the past and it's pretty fast.