Say I want to license a piece of code. I package libraries *with* this code, so others don't have to go through the hassle of downloading these libraries. These libraries are unlicensed. Can I still license my code, and leave these libraries untouched, or how does this work?
Obligatory disclaimer: I am not a lawyer, this is not legal advice, if symptoms persist see your doctor, etc.

Yes, assuming you have the right to distribute the other libraries. The license would apply specifically to the code you're distributing that isn't the libraries - you'd want to make that clear in the license.

If the other libraries are just 'things from the BYOND hub', I doubt anyone could make the argument that you don't have the right to redistribute them stick.

In short, I think legally, you're in the clear, for what it's worth.
In response to Jp
Yes, they were just libraries from BYOND. What you said was my assumption, but I did want to double check. I'll carry on with that, then. Thanks.
I am not a lawyer.

While you can argue that there may be an implied license for libraries on the BYOND hub, it's hard to say for sure- this is more complicated by the fact that the hub is not an unrestricted distribution platform and does faciliate subscriptions. What if I uploaded my library to the hub and meant to tick the subscription box but forgot? I certainly didn't mean to imply that anyone could download and distribute my code.

From what I've seen, if you don't know the license (or distributing it may break the license) your best option is to provide a link (i.e. a hyperlink, don't dynamically or statically link to it!) to the libraries you use with your project. This is what the Qt framework does for MySQL support. Because Qt is dual licensed (LGPL and their own commercial license), they can't freely use/distribute the MySQL libraries (because MySQL only grants a license to use/distribute to projects that have only a FOSS license). What Qt does is give instructions on how to download and build in support for these libraries.