I excavated one of several wooden coffins with viewing plates from the Uxbridge
Almshouse Burial Ground in Worcester County, Massachusetts.  Dates for the
assemblage were roughly 1830-1870.  The report is
Elia, Ricardo, and Al B. Wesolowski, editors
1991 Archaeological Excavations at the Uxbridge Almshouse Burial Ground in
Uxbridge, Massachusetts, BAR [British Archaeological Reports] International
Series 564, Oxford, UK.
 
Ed Bell later wrote up the coffin-related materials in an article in Historical
Archaeology.  His published bibliography of mortuary archaeology may provide
further information on the topic.
 
As for the legal issues, they will vary from one municipality to another, and
from one state to another.  Often local ordinances will protect what state laws
and SHPOs will not.  Incidentally, public cemeteries very seldom legitimately
fall into private hands.  There is often a clause inthe deed "reserving" them
from sale.
 
L. J. Cook