Ah, no, I figured it out. You can make Lion re-appear in your Purchase tab by option-clicking on the Purchase tab icon. I did that some time ago and now Lion is permanently visible for me.
Who thinks of something like that? If you do something stupid like that, at least support should know about it. For shame Apple, for shame! m(
I remember already being annoyed about that some time ago.
At this point it's academic; the laptop is gone... but if I did something wrong I'd definitely like to know :).