Setting boundary conditions on interior cells

This is probably a bad idea, but I want to set a boundary condition on  
an interior cell. Alternatively, I would just have a rectangular mesh  
with some of the nodes masked off (kind of like a step on one of the  
rectangle's sides). The problem with changing the mesh in this manner  
is that this boundary condition needs to move in time.

My first thought was to force a cell to a given value by repeatedly  
resetting its value inside the solution loop. But, since the diffusion  
terms use the face values (not cell values) in the solution, I'm not  
getting the value back. The next (bad) idea I had was to set the face  
values, but I didn't see a way of doing this.

Is there a way of setting a boundary condition on an interior cell?

Thanks,
-Tony

Test script:
~~~~~~~~~~~~~~~
from fipy import *

dt = 0.001
# material properties
D = 1.
c0 = 1.
c_sink = 0.
# spacial parameters
nx = ny = 3
dx = dy = 1.
mesh = Grid2D(dx=dx, dy=dy, nx=nx, ny=ny)
x, y = mesh.getCellCenters()
bottom_center = (x == 1.5) & (y == 0.5)

phi = CellVariable(name='concentration', mesh=mesh, value=c0)
phi.setValue(c_sink, where=bottom_center)
eq = TransientTerm() == ImplicitDiffusionTerm(coeff=D)

all_faces = mesh.getExteriorFaces()
BCs = (FixedFlux(faces=all_faces, value=0),)

for step in range(3):
     phi.setValue(c_sink, where=bottom_center)
     eq.solve(var=phi, boundaryConditions=BCs, dt=dt)
     # one of the cells should be 0 (c_sink value)
     print phi.reshape((ny, nx))