Time Machine stopped backing up my iPhoto library a year ago, lost all photos I recently got my iMac's HDD replaced and when I restored my computer, every single Time Machine backup stretching back to last October contained the same size iPhoto library (+/- 100MB), which was current up to last February, even though the rest of my backups were intact.

Even stranger, before I had the HDD replaced, I copied the Master's folder in iPhoto's Library package to an external drive, and it was also incomplete, and in exactly the same way.

I managed to salvage some photos that were in Photostream by going into ~/Library/Application Support/iLifeAssetManagement/assets/sub in the Time Machine backup, but many are missing and all my videos are gone (because Photostream doesn't do videos.)

Is there anywhere else my photos could be hiding (an import cache, iPhoto trash folder)? Is there anything at all I could do? How do I notify Apple about this issue?
This sounds like somehow your iPhoto Library has moved. If you hold down ALT when you click to start iPhoto, the library management dialog comes up. This will tell you where iPhoto is currently located. If you somehow had access to your old HDD, you could see where this was located.

Obviously iPhoto doesn't work without a library, so the library has to be somewhere. I would certainly dig a little to see if you could find it. 



 Unfortunately, I haven't worked with HDD recovery at all. I do know there are companies that do this. You could potentially send it it. I'll update or comment if I do find something.