Can an invisible Higgs boson be seen via diffraction at the LHC?

We study the possibility of observing an `invisible Higgs boson in central exclusive diffractive production at the LHC. We evaluate the cross section using, as a simple example, the Standard Model with a heavy fourth generation, where the invisible decay mode $H to u_4 bar{ u}_4$ dominates, with the heavy neutrino mass $M( u_4) simeq 50$ GeV. We discuss the possible requirements on trigger conditions and the background processes.