You can deploy a modified client with a rule that declares some coins invalid, as a part of a new protocol, which would result in a hard fork, correct.
That could be done with a soft fork. Miners have incentive to enforce it since it would prevent unpredictable inflation.
You're right, that would make it easier - and more difficult to change back to the old protocol (because restoring the validity of Satoshi's coins then would need a hard fork as new blocks without the invalidation would be incompatible with clients enforcing it). It would basically be the ETH/ETC scenario.
However, with enough consensus the change can always be reverted. There could even be a hybrid client solution, where users who disagree with the new restriction could simply not enforce the rule unless they're mining. That would make a change back to the old protocol easier: They could include a mechanism similar to a soft fork, but which simply creates a hard fork back to the old protocol if enough of the previous blocks (e.g. 90%) support a change back.
The protocol without Satoshi's coins could obviously continue to exist, resulting in two different chains, and the question then would be if "protocol purity" or "lower coins supply" would be more attractive.
(Edit:) A plausible scenario would be the following, in the case there's a soft fork invalidating Satoshi's coins:
- Satoshi announces that he will donate most of the coins to charities.
- He creates and broadcasts transactions to known addresses to the charities, with a large part of the outputs blocked by long timelocks, so the charities can't dump the coins at once.
- In parallel, he distributes the alternative client which would fork back to the old protocol and removes the "invalidation", like I wrote above.
- Users wouldn't have to fear a large dump anymore, so the main reason for the invalidation would not be true anymore. That would make it attractive to use the client with the restored protocol. Satoshi would not have even to come out of his anonymity, because the signed and broadcasted transactions from his accounts are enough for the users to believe him.
- This would result obviously in a communication war, but if Satoshi isn't too greedy I think he would have a big chance to succeed and that in the end the chain with the "backfork" to the old protocol would be the stronger one. If user support is strong enough, miners will also find it more attractive to support the "new old" chain.
(Another Edit:) Satoshi could obviosly also simply bribe miners with his coins, and/or combine both methods.