Let's say I have a legitimate longer chain than the one they plan to lock-in...
Who do I appeal to?
You could appeal to the developers who plan to lock in the other chain. If it's due to a bug or other mistake, they will naturally fix the problem. However, if it's due to evil intent, see your next question.
Do I report to the police? The blogosphere?
There's not (yet) a law relating to the lock-in of chains, so I doubt the police will be interested (or even understanding).
You can report the problem here, and you could report it to the blogosphere if you like.
Who will take the effort to verify my claims?
Anyone who shares your interest in a valid block chain will be motivated to make the effort, and those who are technically able to do so will be happy to look into your claims
provided they are expressed in such a way that people are motivated to take them seriously.
It's in your interests to explain the problem clearly, and it's in other people's interests to take your report seriously (because their bitcoin balance depends on a valid block chain too).
What will be the consequences if I'm proven right?
Someone will release a version of the client software that works properly, which you and others will use. It's of no consequence to you if there are still some people who choose to use the "corrupt" version of Bitcoin, because you are free to use the version with the correct block chain.
I was starting this investigation to decide whether to invest big money in bitcoin. Fat chance.
No problem. We will welcome you back when you are ready.
I was probably a member of the open source community before you were born.
Then you'll understand how it works, and you'll understand why you don't report bugs to the police, and you'll understand that sourceforge is a tool and not a central authority.