Assessment of learning, as the term (and the practice) is currently understood,implies a set of objectives that the teaching/learning process is designed to achieve.
Emergent learning will not be assessed, nor can it be assessed since its very nature implies an absence of clear objectives that drive the process.
Thats the easy answer. But in fact frameworks and contexts can be designed in such a way that learning or learnings can emerge, (the young chimp mentioned earlier may be a very interesting example of this, it would be useful to unpack intentionality there) and the space between emergent and planned learning can be conceived of as a slow cline. Different assessment mechanisms can be brought to bear, and instances of emergent learning can be placed under the light and examined, by the self and others.
Perhaps a useful idea here is ipsative assessment. Measurement is done against the benchmark of the self. Value emerges quite simply.
Best to all
Nick