I would not ride anywhere long distance in jeans, thats just plain bad advice.You should have full safety gear on, a boot, pant with protection, protection in knees, jacket with elbo, shoulder and back protector, and full face helmet. Its all two lane roads overall, no shoulders, and rain in higher elevations in rainy season, which starts in April in Costa Rica.
If you see the way people drive in Central America, the more safety gear the better. Getting to a decent hospital could take hours and hours...
There is plenty of riding gear made for all the elements and temps. Alpine Stars. Klime etc,even Tourmaster and Icon, is better than nothing.
Adventure Travel to Central America, SE Asia, North Africa, EU and USA National Parks.