Hi Walter,
Thanks for the suggestion, but I don’t believe this is a good solution for me since there is really no guarantee that join nodes are indeed on layers with less nodes (guaranteed only in the very local branch of the tree/graph).
I think you helped someone achieve something similar in GoJS here: Center Children in Layout - #4 by srinivasdonapati but the link for the code there is broken. Also the code is of course not WPF.
If there is no simple method to override in our specialization of LayeredDigraphLayout, I might have to revert to using the TreeLayout (our graph is basically a tree with join nodes) and manually calculate the space needed for each branch from the bottom up. Any other ideas before I do that?