Currently there only is the long-click feature for app-icons in the taskbar. I dont know if that changes in Tablet mode, but I guess this is essential if there is no other way to emulate right-click systemwide.
This isn't something we can do generically (in KWin, for example) because it would override any custom press-and-hold behaviors in apps and Plasma parts that we *don't* want to trigger context menus. For example, press-and-hold on the desktop to enter edit mode. As such, press-and-hold-to-open-context-menu needs to be implemented in every place we want it. Please file a new bug report for every such place you find that doesn't have this behavior but you think could benefit from it. Thanks!