How can I force an update on the Viewer Node in 2.80?

I have a rather complex situation here, where I am triggering changes within a node group in the compositor by changing other properties in the image editor. This works just fine in general, but when that node group is connected to a Viewer node, and the image editor displays that Viewer node, the resulting image only updates if the compositor is also visible. If the Image editor is maximized for instance, the updates are skipped.

How can I force the Viewer node to update from within the image editor via Python?

Maybe even more general, is there a way to force a general depsgraph update via Python?

I’ve also postet this question on Stackexchange, including a sample file and a how-to. Might make investigating this easier.