The nice way of doing this would require better subdomain support than we have right now. However there is a slightly hacky way of doing it which I think will cover your case nicely.

If you take a look at the DirichletBC class (in bcs.py), you'll notice that the set of nodes at which the BC should be applied is calculated in DirichletBC.nodes . So you could simply subclass DirichletBC and replace nodes with a function which returns the index of the zero node. For example (and I confess this is a sketch code which I haven't tried to run):

class PointDirichletBC(DirichletBC):

    @utils.cached_property

    def nodes(self):

        # Find the array of coordinate values.

        x = self.function_space().mesh().coordinates.dat.data_ro

        # Find the location of the zero rows in that 

        return np.where(~x.any(axis=1))[0]

Does that work for you?

Cheers,

David