If I add Products.CMFPlone as source checkout in mx.ini with [Products.CMFPlone] it is correctly commented in the constraints list but added as "non version" at the bottom ... other packages get added commented with "source code wins" ...
constraints-mxdev.txt:
...
# # Products.CMFPlone==6.1.0a5 -> mxdev disabled (source) -> mxdev disabled (override)
...
###############################################################################
# mxdev constraint overrides
# plone.staticresources IGNORE mxdev constraint override. Source override wins!
Products.CMFPlone
I use version mxdev=4.0.3
maybe related to this fix: #45