output function of the tangential component of the velocity over mesh boundary

Question

I want to output a certain function to .pvd file at the end of my code. It is a function of the tangential velocity component over a surface of my irregular 3D mesh i.e. a function of T(u) where T is the tangential operator:

def T(u):
return u - inner(u,n)*n

with n = FacetNormal(mesh).

Is this easy/possible? If I just try and project it over the function space I get:
ufl.log.UFLException: Integral of type cell cannot contain a FacetNormal.
which isn't surprising given a normal is only prescribed over the facets. Can I assemble the function over ds (the surface I want the function over) and then project over the 3D function space or something along these lines?

Thanks for your help.