Unfortunately, the DbProviderFactory mechanism is very poorly designed IMHO. The mechanism itself is not extensible, all the interesting classes are sealed (SqlClientFactory, etc.), the configuration in the .config file is not extensible either.

One simple solution is to build an extension class, like this:

public static class DbProviderFactoryExtensions
{
    public static DbParameter CreateParameter(this DbProviderFactory factory, string name)
    {
        DbParameter parameter = factory.CreateParameter();
        if (factory.GetType().FullName.IndexOf("sqlclient", StringComparison.OrdinalIgnoreCase) >= 0)
        {
            name = "@" + name;
        }
        else if (factory.GetType().FullName.IndexOf("oracle", StringComparison.OrdinalIgnoreCase) >= 0)
        {
            name = ":" + name;
        } // etc...
        parameter.ParameterName = name;
        return parameter;
    }
}

It's not very smart, but you can do this in your code:

    DbProviderFactory factory = DbProviderFactories.GetFactory("MyDb");
    DbParameter parameter = factory.CreateParameter("myParam");
    // parameter.ParameterName will be @myParam if SQL, etc.

Or you could also recreate your own system. It's not very complicated, and can be worth it if you need other variable items per type of database.