Would it be allowed that I write my own rendering code beforehand and do the actual content of the game within the 48-hour timeframe? I only ask because I see you allow Unity and GameMaker, and I was hoping I might be able to use my own code as opposed to something else pre-written.

i believe so as long as it doesn't have any features for the game it self

Yep, no problem with that!

Great, thanks!