Shouldn't https://github.com/plone/buildout.coredev/blob/5.1/versions.cfg#L290 be added to http://dist.plone.org/release/5.1.6-pending/versions.cfg ?
5.1.6 will be sort of outdated on arrival, the versions were picked in January and it's May now.
Are the further changes in the Barceloneta 2.X package for Plone 5.2 still compatible with 5.1?
The versions in coredev 5.1 seem pretty up to date 
So far Barceloneta works for both Plone versions and 5.1.6 is still pending