  We present a holographic dark-energy model in which the Newton constant
$G_{N}$ scales in such a way as to render the vacuum energy density a true
constant. Nevertheless, the model acts as a dynamical dark-energy model since
the scaling of $G_{N}$ goes at the expense of deviation of concentration of
dark-matter particles from its canonical form and/or of promotion of their mass
to a time-dependent quantity, thereby making the effective equation of state
(EOS) variable and different from -1 at the present epoch. Thus the model has a
potential to naturally underpin Dirac's suggestion for explaining the
large-number hypothesis, which demands a dynamical $G_{N}$ along with the
creation of matter in the universe. We show that with the aid of observational
bounds on the variation of the gravitational coupling, the effective-field
theory IR cutoff can be strongly restricted, being always closer to the future
event horizon than to the Hubble distance. As for the observational side, the
effective EOS restricted by observation can be made arbitrary close to -1, and
therefore the present model can be considered as a ``minimal'' dynamical
dark-energy scenario. In addition, for nonzero but small curvature
$(|\Omega_{k0}| \lsim 0.003)$, the model easily accommodates a transition
across the phantom line for redshifts $z \lsim 0.2 $, as mildly favored by the
data. A thermodynamic aspect of the scenario is also discussed.
