I see the problem, but...
...I still don't see why transparency can't be allowed on visited links. (I suspect the "won't" could be a typo in the linked article.)
Some info I found speaks about transparency...
Like here: W3.org: SVG Content Interactivity
For raster images, hit detection is either performed on a whole-image basis (i.e., the rectangular area for the image is one of the determinants for whether the image receives the event) or on a per-pixel basic (i.e., the alpha values for pixels under the pointer help determine whether the image receives the event):
...and how it can be used: Mozilla Developer Center/CSS/pointer-events
The CSS property pointer-events allows authors to control whether or when an element may be the target of a mouse event. This property is used to specify under which circumstance (if any) a mouse event should go "through" an element and target whatever is "underneath" that element instead.
From David Baron's Proposed Solution:
... If the relevant link is not visited, this function returns the color from the first (normal) style context. If the relevant link is visited, it returns a color whose R (red), G (green), and B (blue) components come from the second style context (the style-if-visited) but whose A (alpha) component comes from the first.
Also note what he suggests about mouse events:
If, in the future, we add (highly requested) values to this property that allow mouse events to reach elements depending on whether or not parts of an image or parts of the element are transparent, we need to be careful in two cases. First, SVG filters allow swapping of alpha and color components. Second, if we allow background images (above), those images might have transparency in different places.
These problems could be avoided in one of two ways. We could ensure that pointer-events always looks at transparency based on unvisited styles. Or, alternatively, if we don't allow background images, we could ensure that pointer-events looks at transparency prior to processing of SVG filters (which might be easier anyway).
Conclusion: For accessibility I still think also the transparency could be allowed on :visited for the visual appearance of an already loaded background to show (could also be allowed to change position).