  Recent years have seen much activity in searches for dark-sector messenger
particles in the 10-100 MeV mass range, especially in view of a potential new
light boson conjectured by the ATOMKI collaboration, X17. Under the assumption
that the messenger particle has definite parity and either zero or unit spin,
quite stringent bounds already exist on its coupling to electrons and protons.
Equally stringent bounds on the neutron coupling do not exist yet, but are
nonetheless desirable. We explore how measurements of deuteron
photodisintegration with a quasi-free neutron can yield bounds on the neutron
coupling, and compute projections for a potential measurement at the lowenergy
high-intensity electron scattering experiment MAGIX@MESA. The projected bounds
are found to be competitive for an axial-vector or pseudoscalar scenario, but
not for a vector or scalar scenario.
