I tried creating the launcher as described, and it worked great. It took me a minute to figure out how to select an icon. The board on a spring (or whatever that default icon is) didn't seem intuitive for my wife. I discovered that simply clicking on the icon displays a default directory of icons.
The problem came when I then tested how she'd use the cool new panel icon. On these MacBooks the Panel is pretty crowded, so she'll use the Indicator Applet to turn WiFi off. I toggled the Indicator Applet's "Enable WiFi" Networking option to turn it off. The new icon stopped working and I got this error message:
Error: Connection activation failed: No suitable device found for this connection (device enp2s0 not available because profile is not compatible with device (mismatching interface name)).
So after some struggle I got this working in the Terminal:
notify-send "Joining myWiFi." ; nmcli networking on ; nmcli radio wifi on ; sleep 3 # ; nmcli con up "myWiFi"
It even worked when I pasted it into the custom launcher's Command box. I could use the Indicator Applet to turn off Networking and WiFi, then click the new custom icon and it popped the notification & connected to myWiFi.
Then it didn't work anymore.
So next I made a script named .joinMyWiFi.sh, put it in my Home directory, turned on Execute permissions, and used the Browse option in the Panel's Custom Application Launcher to select it. Now it works in every scenario I've tested. I guessed that something went funky about chaining the commands in one line, so the file contains this on separate lines:
notify-send "Joining myWiFi..."
nmcli networking on
nmcli radio wifi on
nmcli con up "myWiFi"
At this point it seems possible it might actually work tomorrow when I test it some more. 