Best idea I have is to get some full mesh motorcycle over-pants to wear over the padded shorts you have. That way you have some airflow over your lower body, and some protection from the ground. Seems like that would not be comfortable though
Personally, I wear board shorts with Under Armour athletic briefs underneath and a technical t-shirt when riding. All the local trails are in forests, where there is no wind, very high humidity, and high heat. Any more clothes than I already wear would be way too much.