Hey Tom,
if only the Onefinity Z assemblies had more travel! I liked our discussion in this thread and imagine with a Z slider like this, that is able to protrude downwards beyond the lower end of the Z assembly, you would be able to keep the spindle center very close to the X carriage. But I would prefer to have two bearings per rail instead of one, and I fear that even the slightly higher Z-20 slider would not satisfy because its height is not enough for that.