How to generate migration scripts in EF6 with custom SQL?

Question

I'm trying to generate migration scripts in EF6. However, due to EF6's shortcomings I need to call some of SQL manually. It looks like this:

public override void Up()
{
    // ...

    using (var stream = Assembly.GetExecutingAssembly().GetManifestResourceStream("<name>.sql")
    {
        var reader = new StreamReader(stream);
        string sql = reader.ReadToEnd();

        this.Sql(sql);
    }
}

When I'm doing migrations with Update-Database, it generally works as intended. However, the problem shows when I'm trying to generate migrations as scripts. Upon calling:

Update-Database -Script -SourceMigration <name> -TargetMigration <name>

Entity Framework:

• Correctly generates SQL resulting from the code in the migration (CreateTable etc.)
• Correctly generates first line of the custom SQL to the output script
• Dumps rest of the custom SQL to the Package Manager window.

How can I solve this problem, so that the full migration script is generated to the output file?