A 'hard fork' implies a functional event in the Bitcoin network having to do with block linkage.  You are getting tunnel vision.  I'm saying that exercising control over Bitcoin (including forced spending) is better done in a different way.

When BTC is used by a certain percentage of the public and corporate entities accept BTC the 'solution' becomes simple"

 - Some 'terrorist' uses Bitcoin for something bad.

 - The government tells corporate entities to check against a blacklist (maintained by Mellon's company) before accepting payments.  (You can ask Yahoo!'s CEO what kind of pressure can be brought to bare to make national security things happen BTW.)

I personally am as political as they come, but I'm not going to accept BTC which are on Mellon's list because I don't know if I can get rid of them without a loss.  I don't mind taking a loss on principle, but I want to know what the loss is going to be.  My point is that independently maintained blacklists will be very effective for reasons that have to do with generic economic principles.

Now that blacklists are implanted, it's just a matter of ratcheting up the things that trigger taint.

 - Starts with 'terrorism' and 'child exploitation.'  ('terrorism' includes Wikileaks of course.)

 - Next it's violent crime and drugs.

 - Next it's fraud.

 - etc.  I mean, who's going to stand up and say that fraud is OK?  It's a progressive 'frog in the pot' type operation.

Of course some people slip through the cracks, and that problem can be mitigated by using the sister 'identification authority' so blacklist any coins the don't have a known owner.

Now we get to the OP.  For legacy reason, older BTC stashes are not associated with an individual owner.  Solution is trivial:  Force a spend and blacklist BTC which don't move.