Custom Assembly Attributes

Yes you can. We do this kind of thing.

[AttributeUsage(AttributeTargets.Assembly)]
public class MyCustomAttribute : Attribute {
    string someText;
    public MyCustomAttribute() : this(string.Empty) {}
    public MyCustomAttribute(string txt) { someText = txt; }
    ...
}

To read use this kind of linq stmt.

var attributes = assembly
    .GetCustomAttributes(typeof(MyCustomAttribute), false)
    .Cast<MyCustomAttribute>();

Yes, use AttributeTargets.Assembly:

[AttributeUsage(AttributeTargets.Assembly)]
public class AssemblyAttribute : Attribute { ... }