I finally heard back from NI this morning (okay, I admit I didn't actually ask until this morning) and this is an issue they're aware of and it does seem to be Mac specific. Here's a
page where they explain the problem. Until they fix it (I get the feeling they've been stumped for a while on this one, so it might not be soon), you do indeed need double the size of the library available on your system drive, even if you select a non-system drive as the destination.
I see three options:
1. Temporarily copy something big from the system drive to another drive to clear up space. Install the mystery library, then move your porn folder back to the System Drive and the Mystery Library over to your Samples drive. (Presumably named "Rectum Samples"?)
2. Nag the lazy developer to upload rar files of the library. He's probably already planning to do that, but it's a long process, because his rar app is on an old laptop, so it's kinda cumbersome.
3. Install it on another computer and copy it to the good computer. (Well, "good" except for the puny system drive.)
Note that authorizing the library is separate from installing the library. You have to authorize, but you don't have to use NA to install. So Options 2 or 3 will work, even if your system drive has zero free space, since you can bypass that step. In other words, once you've copied this mystery library to your Samples drive using Options 2 or 3, then open NA, add the serial, skip the "Install" process, and it will ask you to point it to the library's folder.