I think you've asked an important question here... I wish I better understood the nuances of licensing and how all that really
works, but I'm totally out of my depth wrt that sort of thing.
All of my uncertainty/apprehension around this issue is part of why I wanted to do that ludum dare entry from memory. (Of course the main reason being it's just better to practice that way -- not to mention dems da breaks of LD anyway). But even that is kind of a grey area to me, because even from memory you end up with things that can be very similar. How similar is too similar? Etc. etc..
I know Casey addressed this on the stream at some point (probably in a Q&A) but I'm afraid I'm not sure which session. The sentiment though, AFAIR, was something like what's on the main page / FAQ:
How will the source code be licensed?
Two years after the final version of the game is released, 100% of the source code will be released into the public domain. Prior to that, all rights are reserved, so please contact me for permission before you use it in your own projects. This is a precaution to prevent people from directly cloning the game until after it has been out for a reasonable time, since this project is an expensive endeavor and I hope to offset some of the cost through sales of the game proper.
There's also a license file included with the HH source code with some of the finer points (I won't attempt my layman's interpretation).
As chebertapps said, it's probably best just to run things by Casey first.
P.S. I'd love hear anyone well-versed in how licensing usually works chime in here. Any sort of general "intro to sharing code" resource recommendations?