Failing to send Wake-On-LAN UDP packet with netcat
I've likely answered my own question:

The BSD version of netcat included with macOS lacks the -b (broadcast) flag.

And even though 255.255.255.255 is a special global address for broadcasting to the entire subnet, simply sending to it apparently isn't enough to imply/accomplish a broadcast without that missing -b flag.

So, my impression is that the macOS version of netcat simply won't do what I'm trying to do.

If I were to install an alternate netcat implementation using homebrew or some other source, it might accomplish it--but at that point, I'd rather just depend on having wakeonlan installed.

I'll leave this answer here, but unaccepted, hoping that someone with more low-level networking know-how provides a better answer at some point.

Edit: Also, if one were going to install other software due to netcat limitations, it looks like socat (described as "netcat on steroids") would likely be something to seriously consider, instead of just an alternate version of netcat.