I had the same problem when I first switched from boots to trail runners. I never even thought about the exposed ankles. My socks were much thicker (Smartwool PhD) but the little mosquito probosces still went right through! After my first trip with trail runners, I drove 850 miles home from Wyoming to Oregon trying NOT to scratch my ankles!
A permethrin spray treatment will last through 10-12 machine launderings (with detergent and thorough rinsing), so wading creeks and sweat shouldn't be a big deal unless it's a really long trip. Personally, I go with lightweight breathable gaiters, which also keep pebbles out of my shoes. Since you reject both of these alternatives, I don't know what else to suggest, except maybe Deet repellent on your ankles. Any socks that would be mosquito proof would also sweat your feet and ankles like crazy.