I read in the FF that there will be a selector combinator in 2.0 and that seems to be exactly what you want to do the routing for deciding where the cube should go on a bit more high level. There is a mod that implements it already similar to how it will be in 2.0:

https://mods.factorio.com/mod/pigo-selector-combinator

It would perhaps be nice to implement in tech tree when enabled so it is accessible a bit later after building the first mainframe, using plain signal forwarding and XOR etc.

I tried it and it just showed up as unresearched so I suppose it needs to be integrated?