I think that's a bit off base here. It's like asking how does a keyboard user facilitate click-and-hold to pan the canvas in a drawing program? They don't. Panning with the mouse (or dragging objects) is one interface. The task is panning the canvas, the means to achieve that are varied.

In my example, the answer to the question about accessibility is to additionally provide keyboard controls to complete the task (preferably ones that don't - or optionally don't - require holding a key). For example perhaps a key shortcut to enter a pan mode, in which the arrow keys move the viewport around the canvas. Problem solved.

As for draggable objects, the task is re-ordering. How do you make that accessible? Provide an alternative means to re-order objects, perhaps using TAB to cycle focus through the objects, then a key to select the focused one, and use the arrow keys to move the drag preview to the nearest valid position in that direction.

Overwhelmingly, the answer is "you don't." Your UI should provide alternatives to dragging that allow folks who can't use a point/touch devices to interact with your page. Which is to say, don't shoehorn keyboard support into your drag implementation, add separate keyboard functionality that makes sense in addition to drag functionality.

That would be a moveable object, and I reckon it would be easy to implement using this as a basis.

tab / space / enter to select an item, crtl + arrow keys to move the "dragged" element. Obviously, it can only move one block per keystroke.