Thanks for that, rrao! It worked mostly as advertised; the button is added to the toolbar when the view changes.
All is not well, however... the button is supposed to trigger a Dwt dialog. When I click the button added by onShowView, the dialog is never created; Firebug throws a "args[0] is undefined" error (Dwt.
js line 1092 --- if (args.length > 1 || args[0]._eventMgr) { ) in response to
var view = new DwtComposite(this.getShell());
The button does still work normally in the view that was present at startup (e.g if the client starts in conversation view, it works in that view but not in traditional).
Any help or insight you can provide would be most appreciated. I can post the entire code if it helps.