FWIW, ‘#’ appears to work as a comment character.
It at least has the effect of removing unwanted environment declarations. It might be declaring others starting with a #, but… it still works.
EG
DATABASE_URL=postgres://mgregory:@localhost/mgregory
#DATABASE_URL=mysql://root:secret@localhost:3306/cm_central
results in postgres being used by django when started by foreman with this .env file, which is what I wanted.