I realise this is a very old and now dormant thread, but I thought I should add somthing seeing as all colleges do not follow the policies listed above. Trinity is happy for anyone to stay up over the long vac so long as you let them know that you are doing so. I and a friend are still up - he is punting for the Granta, and I am working at the UL. The college does reserve the right to make you move rooms (though they can only do this if you are not a scholar), but it is very rare that this will be required.
However, this would not be possible before matriculation. Indeed one would not be allowed into the UL until one had a student card.