Compound ortho edges through clusters

I have a set of cluster subgraphs and I want to impose some hierarchy on them.
I have compound=true and some ltail/lhead edges between (nodes within) them.
When splines=true or splines=polyline, the edges avoid other clusters between their head and tail.
When splines=ortho, the edges tend to pass through clusters instead of going around them.
It seems like the avoidance should be similar for ortholines and splines.
I would like to use ortholines if I can for these inter-cluster edges, but I would rather they did not pass through clusters.
Can that be done?

EDIT: If you need an example:

digraph G {
compound=true
splines=ortho

subgraph cluster1 { a1 a2 }
subgraph cluster2 { b1 b2 b3 b4 }
subgraph cluster3 { c1 c2 }
subgraph cluster4 { d1 d2 }

a1 -> b1 [ltail=cluster1 lhead=cluster2]
b1 -> c1 [ltail=cluster2 lhead=cluster3]
a1 -> c2 [ltail=cluster1 lhead=cluster3]
a2 -> d2 [ltail=cluster1 lhead=cluster4]
b2 -> d2 [ltail=cluster2 lhead=cluster4]
}

EDIT 2: It comes to mind that ortholines are maybe only designed to have 3 perpendicular segments maximum. That should not prevent my above example from being able to generate a noncrossing ortholine from cluster1 to cluster3 not through cluster2, but I also guess the algorithm is trying to generate the shortest one? It seems like these requirements make my request impossible in general. Is that the case?

Alas, the problem has a

Alas, the problem has a simpler explanation. The implementation of orthogonal routing was never fully completed. It doesn't handle node ports, edge labels in dot, nor clusters. Let me think if there is a simpler workaround for your case.

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