Rather than get into an argument over semantics, in the context of this discussion, I suggest that the terms “design” and “specific design requirement” would be descriptions of the physical characteristics of a system; “emergent behaviour” would be a qualitative description of the observed actions of that system.
Continuing with the sponge example of emergent functionality: it seems to me that there are three critical issues that have bearing: the physical process of transferring newly acquired capabilities to descendant organisms; the reproductive capabilities of emergent organisms; and the life span of both individual cells and the associated organism.
Assuming that the specialised blob resulted from an aggregation descended from a single mutated cell, how was the subsequent aggregation reproduced into the next generation of the containing organism? Your description of the sideways exchange of DNA is understood, and relevant in current developed organisms, but of dubious importance in emerging organisms.
In the context of the emergence of new life forms, there are three constraints that would work in opposition to this process: the absence of a defined mechanism of data transfer; the volume of life forms; and their life span.
The mechanism of transfer implies that all of the following be in place: a mutated aggregation of cells contained within a larger organism; the capability of physically transferring the newly acquired functionality to a replicating functionality; the existence of a duplicate and receptive organism; physical proximity of both donor and recipient organisms; a process for the transfer of specific genetic material to specific target cells; a means of incorporating and subsequently expressing the newly acquired functionality into subsequent offspring.
It is evident that at the time of emergence that there would be insufficient number of mutated cells to accomplish the kind of DNA transfer that we observe in currently existing life forms; they would have statistically insignificant number of targets, and both the donor and recipient would probably die before the process could either be effected or have any beneficial effect.
Back to the kidney example: in that the lifespan of the individual cells is necessarily short, the death of the innermost cells and the resultant cavity would be unlikely to convey ongoing benefit: both the remaining cells and the organism itself would have died before the organism could lock in the structure, develop a mechanism of functional transfer, or enact the process described above.
Furthermore, for the cavity to have any long term functional benefit, it would necessarily need to acquire functional specialization: the cavity would require a lining of cells compatible with the wastes to be conveyed; the lining cells would be required to develop functional specialisation; and the system would require the development of directional specificity; ie: the wastes would need to be conveyed out of the organism, as opposed to flowing in random directions within the organism.
The time required for these design requirements to evolve would be far greater than the lifespan of both the individual cells and their containing organism; and the organism would have been poisoned by its own wastes before any such structures evolved.
Regarding “The Game of Life”, I have observed and been fascinated by it in action in many of its variations; however, in the final analysis, I have to put it into the same basket as String Theory and Climate Modelling: interesting mathematical constructs, but of little relevance to the real world.
Similarly, we cannot continue to skate around the issue of biogenesis: without a comprehensive and detailed description of the mechanisms and processes of biogenesis, and specifically the emergence of DNA, all of the above discussion is little more than the equivalent of the counting of the number of angels on the head of a pin.