I know dry-hopping while primary fermentation is going on is a futile effort
Why do you say that?
"(Matt) described a process where the hops were introduced in the primary fermenter and allowed only a specific contact time before being racked off. He stated that the action of fermentation was needed to scrub some of the undesirable flavors from the hops and leave the fresh hop character that was desired. As soon as the head of the primary fermentation starts to crash, this is the time to dry hop. (approx 3-4 days into fermentation)"
Credentials: Matt started working in the lab of the Kalamazoo Spice Extraction Company, and he was soon sent over to Chicagos Siebel Institute, where he continued his education in organic chemistry.
Sounds like he knows what he's talking about, or at least more than us.
The recent (2016) article from west coast brewers (Stone, RussianR, and others) seems to prove that even the pros don't know everything about hop usage. I've read articles advocating pretty much every method conceivable from multiple whirlpool additions, multiple dry hop additions, water chemistry, temperature considerations (stone dry hops at 60F), and countless other variables and combinations.
So, find your own way and what works for you.
Personally, I only late hop (never use hops prior to 30 mins anymore), whirlpool, add at least one dry hop addition and sometimes two, then usually keg hop in a nylon mesh bag.