I think this is not possible. If there was a small angle-preview (maybe in the property bar), then you can do it. You could move your mouse or pen and click when you have reached the right angle. With a modifier key it would be also possible. The best way is together with an angle-preview and the possibility to tell Artweaver in which steps the angle will change with every hit on the modifier key (you can put this in the property bar, too). That would be perfect.
It's the same problem when you want to draw a shape or a custom shape. You can rotate the layer afterwards but not rotate the shape during the process. And if you rotate the layer afterwards, everything on the layer will rotate.