Well, yes... people do it all the time, but that doesn't mean it's not breaking copyright.... and while individual people are free to do what they want (as they accept the consequences of their actions), if we're going to do it as a division thing, we have to make sure whatever we do can't land us or TOG in trouble. Also TOG does state that it does not condone any actions that would break a game's EULA, or discussion/promotion of illegal activity. So yes, it's a general thing and a TOG thing.
Essentially everything someone creates (writing, art, photography, music etc.) is copyrighted to them (automatically), and if someone else uses/copies/reproduces that without their permission, it's a breech of copyright.
There are exceptions to this... anything designated "creative commons" is basically saying that it's ok to copy/use/reproduce (eg wikipedia content, free web graphics and so on)... and there is a provision in copyright law for "fair use" - which isn't actually defined, but basically says that certain uses, for things like review, parody, commentary etc. may be allowed. So that people like movie reviewers can show clips of a movie while talking about it, schools can reproduce passages from books for students to discuss etc.
So technically speaking, the opening sequence (disregarding the sound for a moment) of Halflife is valve's intellectual property. The character names, the facility name, the look and layout of the facility. The same as for the plot of a movie or book. So reproducing that, even without sound, even if it was made in minecraft by us, could be a breech of copyright - because we are reproducing enough of that game that it is recognisable as a copy of it. Adding sound taken from the game is yet another breech.
Realistically speaking.... would Valve be likely to a). find it? b). really care? c). prosecute?... extremely unlikely.... but rules are rules, and I don't think we as a division can do anything that would be a breech of copyright.
Is this something that is allowed under parody?
Depends on how it is done.... if it's done in a funny way at all, - like for example if Barney was a creeper or something... then yes, it could probably be considered parody rather than copying, and then it would be ok. But if it's basically just half life reproduced in minecraft, then it's a reproduction, not a parody.