  We calculate form-factor ratios between the semileptonic decays
\bar{B}->D^+\ell^-\bar{\nu} and \bar{B}_s->D_s^+\ell^-\bar{\nu} with lattice
QCD. These ratios are a key theoretical input in a new strategy to determine
the fragmentation fractions of the neutral B decays, which are needed for
measurements of BR(B^0_s-> \mu^+\mu^-). We use the MILC ensembles of gauge
configurations with 2+1 flavors of sea quarks at two lattice spacings of
approximately 0.12 fm and 0.09 fm. We use the model-independent
z-parametrization to extrapolate our simulation results at small recoil toward
maximum recoil. Our results for the form-factor ratios are
$f_0^{(s)}(M^2_\pi)/f_0^{(d)}(M^2_K) =1.046(44)_{stat.}(15)_{syst.}$ and
$f_0^{(s)}(M^2_\pi)/f_0^{(d)}(M^2_\pi)=1.054(47)_{stat.}(17)_{syst.}$. In
contrast to a QCD sum-rule calculation, no significant departure from U-spin
(d<->s) symmetry is observed.
