Is there any way the Bitcoin network could resist a viral tainted-coin tagging system implemented by regulators?
I don't think the scenario is realistic because it would only punish innocent people. By the time coins could be marked as tainted, they would already be in the hands of innocent people. Governments don't do similar things with other currencies. For example, if you deposit dollar bills in a bank that turn out to have been stolen, the government could charge the bank a "fee" to clean the bills which the bank could pass on to you. But governments don't do that because they recognize that by the time the bills could get on a "hot list", they're already in the hands of innocent people.

There are also specific features of Bitcoin that make this difficult. Transactions put all the input Bitcoins into a big pile which they dole out as outputs. There's no concept of particular output Bitcoins corresponding to particular inputs. Bitcoins don't have a serial number that can be traced through the block chain. So any such tracking rules would be basically arbitrary.

It's just not something realistic. It's more realistic with physical currencies which have serial numbers and are widely used to facilitate crimes and buy contraband. And governments don't do it with physical currencies -- largely because they know it would only punish the innocent.