I know of four possibilities:

1.) Special seats may be already pre-reserved, e.g. for disabled, family with babies, etc. Even if you may be able to get such seats, CX warns that you may be bumped from those seats in case someone in need requires those.

2.) Travelers with certain elite tier status on CX Marco Polo can book their seats even earlier than 48h before departure. You can then select via website or call them. The higher the status, the more seats become available.

3.) Extra leg-room seats, which are indeed more expensive and can be either redeemed with Asiamiles or paid by the traveler.

4.) Travel agencies can reserve seats for you already. At least our corporate travel agency can do that and I believe any other travel agency can do the same.

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You still can't reserve seats more than 180 days before departure via the website. And yes, there are people doing that to catch the cheapest fares for peak-seasons like e.g Christmas or Chinese New Year.

I'm not sure whether above is exhaustive. Hope it helps already though.