That's a very thought-provoking question. Adding placement constraints on top of an automatic layout engine sounds like a challenging problem. You'd have the engine deciding where to place things, but you'd also be nudging things or constraining things.

This might not be what you have in mind, but Graphviz has a `rank` which can be used as a coarse way to influence the shape of the diagram. (I'm assuming you had something more sophisticated in mind though)

Probably easier would be if the relative drawing were its own distinct Mermaid diagram type, but then it would be largely a new language anyway.