[BsonDefaultValue(0m)]  shows the compilation error: Error CS0182 An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type

However if used with an integer in an object plroperty such as:

[BsonDefaultValue(0)]

public decimal Amount { get; set; }

And then subsequently loading a record with no value for Amount from the database,   the error Unable to cast object of type 'System.Int32' to type 'System.Decimal' is thrown.

So it appears that you cannot set a default of the correct type for decimal currently.