The quadratic hashing time issue is a non-problem. Miners employing parallel validation do not fall victim to extended time validating blocks containing aberrant large quadratic hashing time transactions. Instead, they orphan such blocks. By continuing to mine and validate on other threads while the validation of the aberrant quadratic hashing time block runs on one other thread. Miners who continue to make blocks with such transactions will eventually bankrupt themselves. All without doing any damage to the network. Problem solved.

What implementation includes parallel validation? Oh yeah... BU does.