  Using diagrammatic resummation techniques, we investigate the
double-logarithmic series of the "soft-overlap" contribution to $B_c \to
\eta_c$ transition form factors at large hadronic recoil, assuming the scale
hierarchy $m_b \gg m_c \gg \Lambda_{\rm QCD}$. In this case, the hadronic bound
states can be treated in the non-relativistic approximation and the relevant
hadronic matrix elements can be computed perturbatively. This setup defines one
of the simplest examples to study the problem of endpoint singularities
appearing in the factorization of exclusive $B$-decay amplitudes. We find that
the leading double logarithms arise from a peculiar interplay of soft-quark
"endpoint logarithms" from ladder diagrams with energy-ordered spectator-quark
propagators, as well as standard Sudakov-type soft-gluon corrections. We
elucidate the all-order systematics, and show that their resummation proceeds
via a novel type of integral equations. The current status of the calculation,
which includes all double logarithms in the Abelian limit, is reported.
