Is it possible to ddd DayPilot:MenuItem items at runtime ?

That's no problem. You can do it this way:

MenuItem item = new MenuItem();
item.Text = "Delete";
item.Action = MenuItemAction.CallBack;
item.Command = "delete";
DayPilotMenu1.MenuItems.Add(item);

You can assign an event-specific context menu in BeforeEventRender event (e.ContextMenuClientName; should be equal to DayPilotMenu.ClientObjectName).

The method of dynamic menu creation from my previous post doesn't allow you to add/remove items during CallBacks/partial PostBacks (only full PostBacks). BeforeEventRender will allow you to create several menus and assign them as needed. Most likely, you won't need a different menu for each event but there will be just a few types of menus based on event type anyway.

I am experiencing a similar situation (different types of events that need different menus) and thought that this was the best solution. The problem is I am working with the month view calendar. The BeforeEventRenderEventArgs are much more limited for the month view and does not include ContextMenuClientName. Is there another way to accomplish this, or would it be possible to enable this in a future release?

Yes, it will be added to DPM. See http://forums.daypilot.org/Topic.aspx/544/contextmenuclientname_for_month_view.