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.
Given the massive amounts of ram required by ultra large transactions that are heavy in sigops that would be prone the quadratic scaling laws, validating yet another block in parallel is an excellent way of using even more ram. High ram servers with 256GB may be able to cope temporarily with it but normal machines and even normal servers will likely run out of memory and kill bitcoind.
Which implementation has had out of memory issues already? Oh yeah... BU did.