And what is the problem define another maps from segments and colour them?
Having to split my overall map up into yet another series of submaps, each of which covers an arbitrary group of maps from several unrelated subsections, is getting really close to spaghetti code.
I withdraw my request. You were right, using maps I can make this very logical and "clean" (for some value of "clean"). Each survey exports one map for each "plane" of the elevation view. (Optionally, it can also export a combined map, in case I want to render it separately from the other caves/surveys, such as in an atlas.) Each parent survey exports a map for each plane used by any of its children, containing the relevant maps from the children. At the top level, I have the overall map which combines the plane maps. The lookup then only needs to match those. Or they can have the colours hardcoded. Thanks for the hints :) Tarquin