How can set a default value constraint with Entity Framework 6 Code First?

Unfortunately the answer right now is ‘No’.

But you can vote for Better support for default values

EDIT 30 Mar 2015:
It’s coming in EF7… Support database default values in Code First

EDIT 30 Jan 2017: General support for default database values is part of EF Core (the new name for EF7)… Default values

EDIT 17 Jan 2018: I’m not sure why people are commenting that EF7 is still a “pipe dream”. EF7 (renamed EF Core) was released on 27th June 2016 and supports the setting of default values in Code First using the FluentAPI like this:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Blog>()
        .Property(b => b.Rating)
        .HasDefaultValue(3);
}

EF Core 2.0 was released on 14 August 2017