This sounds weird. Are they going to have two versions of every executable? One arm and one intel? Or would this low power/power nap mode only support a tiny subset of functionality that apple chooses to include in their OS release?
Thinking about this I've convinced myself that what will happen is the ARM core will periodically check for various statuses (i.e. connect to your imap/pop server and see if there is new mail) and turn on an indicator or perhaps wake the intel cores. Which is to say that the powernap/low power functionality will be restricted to either things apple delivers ootb or to a special API. Not as something that automatically migrates whatever workload is running on the powerful intel cores into a slow motion version of the same thing running on the ARM cores.