  Exclusive hadronic reactions with a massive lepton pair (l^+l^-) in the final
state will be measured with PANDA at GSI-FAIR and with Compass at CERN, both in
p+p-bar -> l^+l^-+pi and pi+N -> N'+l^+l^-. Similarly, electroproduction of a
meson in the backward region will be studied at JLAB. We discuss here how the
spin structure of the amplitude for such processes will enable us to
disentangle various mechanisms. For instance, target-transverse-spin
asymmetries are specific of a partonic description, where the amplitude is
factorised in terms of baryon to meson or meson to baryon Transition
Distribution Amplitudes (TDAs) as opposed to what is expected from
baryon-exchange contributions.
