  Starting from the semiclassical reduced-action approach to transplanckian
scattering by Amati, Veneziano and one of us and from our previous quantum
extension of that model, we investigate the S-matrix expression for inelastic
processes by extending to this case the tunneling features previously found in
the region of classical gravitational collapse. The resulting model exhibits
some non-unitary S-matrix eigenvalues for impact parameters b < b_c, a critical
value of the order of the gravitational radius R = 2 G sqrt(s), thus showing
that some (inelastic) unitarity defect is generally present, and can be studied
quantitatively. We find that S-matrix unitarity for b < b_c is restored only if
the rapidity phase-space parameter y is allowed to take values larger than the
effective coupling G s / hbar itself. Some features of the resulting unitary
model are discussed.
