A brief update on the hive troubles: We continued feeding today with leftover honey. We made about 8 lbs available for each colony in the hive-top feeders. It was certainly warm enough for them to take it today. We'll see how much we can get them to pack away before it gets too cold.
I spoke to the former president of the local bee club today. He said this problem seems to be fairly widespread throughout Boulder County this year. He has some theories that it may be related to neoniconoids (a type of pesticide) being used on local corn crops. His advice was add a pollen patty to affected hives to see if that will get the queen laying again.
More resources on connections between neoniconoids and colony collapse disorder (CCD):
USDA website article
Bee Culture Article