I just did this with a new tank on our system. It was isolated to prevent any switch go to the whole house system. But with 100% live rock 40% seeded live sand. And the rest of the sand live from a bag. A dose of biospira the next day after filling... the tank never saw a cycle. I would not recommend this method to any one who doesn't have 20 years or so into the hobby, you're just going to cause trouble for yourself.
I just started my cycle on my biocube as well about two days ago, I used cured live rock from a established tank that's been running for 8+ years and I was told that once you got live rock and saltwater together in the tank the cycle starts from the die off of the rock. So I just boosted it with a raw deli shrimp. Best advice I've learned was be patient and not rush it.
Just filled my 29G biocube up yesterday. Used 20lbs. of live rock, a 20lb. bag of live sand and ocean water. It will be interesting to see what results I get when I test the water.