How can I launch a Cinnamon applet from the command line?

I had the same question, and kynan has/anticipated an answer - if a bit brief:

You have to change the applet you want access by shortcut such that it reacts to said shortcut. At least this is what I did.

My aim was to have the "Windows Quick List" accessible by a short cut, so took the "Window Quick List with close button" as template.

In the _init of the applet.js I added the following lines:

        this.settings.bindProperty(Settings.BindingDirection.IN,
                                  "keybinding-def",
                                  "keybinding",
                                   this.on_keybinding_changed,
                                   null);
        this.actor.connect('key-press-event',
                           Lang.bind(this,
                           this._onSourceKeyPress));

        this.on_keybinding_changed();

And the the two handlers are needed of course:

    _onSourceKeyPress: function(actor, event) {
    let symbol = event.get_key_symbol();

    if (symbol == Clutter.KEY_space || symbol == Clutter.KEY_Return) {
        this.menu.toggle();
        return true;
    } else if (symbol == Clutter.KEY_Escape && this.menu.isOpen) {
        this.menu.close();
        return true;
    } else if (symbol == Clutter.KEY_Down) {
        if (!this.menu.isOpen)
            this.menu.toggle();
        this.menu.actor.navigate_focus(this.actor, Gtk.DirectionType.DOWN, false);
        return true;
    } else
        return false;
},

on_keybinding_changed: function() {
    Main.keybindingManager.addHotKey("must-be-unique-id",
                 this.keybinding,
                 Lang.bind(this,
                       this.on_hotkey_triggered));
},

Finally, in order to have the chance to define the hot key customizable I added the following lines to settings.json (also to be anticipated by the above on_keybinding_changed):

,
"keybinding-def" : {
"type" : "keybinding",
"description" : "Shortcut to open/close the windolist ",
"default" : "Super_R"
}

(Please take care to copy the comma too, if you forget it, you'll run into trouble)

For me this worked (so far, without major hiccups), but I'm neither javascript in json savvy, and it was a lot of copy&paste together with try&error.

So, probably this is only a starting point and not yet the ultimate answer.

Short answer: I don't know of a way.

Cinnamon applets aren't binary applications or scripts, but more similar to plugins implemented in javascript with some JSON metadata. Have a look at the system wide installed applets in /usr/share/cinnamon/applets/ for how they look like.