It will work. Ive used straight Hydroton in Hempy buckets with good results.
Never did worry about evaporation and never found evaporation to be a problem but I've covered the top of the bucket in the past in order to cut light to algae that was growing on the surface of the medium. The only time I've had algae problems was when I tried to re-use the perlite/vermiculite for a second grow. I've never had a root rot problem with Hempy buckets.
That being said, I stick to the traditional 3/1 mix of Perlite and vermiculite. It's whats easiest for me. The cost of new medium for every run is negligible in the scheme of things and I give the used medium to a friend who owns a landscaping co. and recycles it all.
"Mark ye, be forewarned; Ahab's above the common; Ahab's been in colleges, as well as 'mong the cannibals; been used to deeper wonders than the waves; fixed his fiery lance in mightier, stranger foes than whales.'"