The ZNC Wiki references a /znc command often (/znc detach, /znc *modname help, etc.). When I initially set up ZNC a while back, I kept getting an error when trying to use this command:

15:19:42 weechat =!= Error: unknown command "znc" (type /help for help)

I eventually gave up and went back to running WeeChat from within a tmux session.

This past weekend, I gave ZNC another go. After a bit of troubleshooting, I found that support for this command seems to be implemented on a client-by-client basis, and WeeChat simply did not support it. However, the usage of /znc is simply a shortcut for /quote znc. Therefore, it is a simple matter of creating an alias in WeeChat (and saving the configuration so the alias is available next time WeeChat starts):

/alias znc quote znc

After doing this, it is possible to detach from a channel using /znc detach #channel_name.