Comment 2 for bug 1726289

        //Main.keyboard.Show(global.get_current_time());
        Main.keyboard.show(global.get_current_time());

I'm no good at JS, but going through the GNOME Shell JS code, this seems to be the change I needed.
With this change in place, sliding for the keyboard from the bottom does show Onboard.

But now leads in the following non-fatal exception:

Oct 23 15:36:29 priyasi gnome-shell[3160]: JS ERROR: TypeError: this.keyboardMonitor is undefined
                                           LayoutManager<._updateKeyboardBox@resource:///org/gnome/shell/ui/layout.js:468:9
                                           wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
                                           set keyboardIndex@resource:///org/gnome/shell/ui/layout.js:566:9
                                           Onboard<.enable/KeyboardShow/<@/<email address hidden>/extension.js:136:17
                                           Keyboard<.show/this._keyboardRestingId<@resource:///org/gnome/shell/ui/keyboard.js:638:56