  We use the Omnes representation to obtain the q-squared dependence of the
form factors f+ and f0 for semileptonic H -> pi decays from the elastic pi H ->
pi H scattering amplitudes, where H denotes a B or D meson. The scattering
amplitudes used satisfy elastic unitarity and are calculated from two-particle
irreducible amplitudes obtained using tree-level heavy meson chiral
perturbation theory (HMChPT). The q-squared dependences for the form factors
agree with lattice QCD results when the HMChPT coupling constant, g, takes
values smaller than 0.32, and confirm the milder dependence of f0 on q-squared
found in sumrule calculations.
