Note that PyNE includes this data in their material library. It might already be possible to get a corresponding OpenMC material since their Material class has a openmc() method that produces XML (which we could parse with openmc.Material.from_xml_element?).
Obviously being able to get this from openmc.model would be way simpler for a user.
This minimal neutronics material package also includes PNNL materials
It also includes additional materials that have densities that vary with pressure / temperature / density / enrichment
Materials like Helium gas, FLiBe, Li4SiO4 can therefore be accommodated and the database has density equations for these materials and others. This allows the to vary their density with temperature and density automatically.