It would be good to be able to have some generic logic available in the kernel - as a default, with knobs to take control from userspace if there is a policy daemon available.
Perhaps something like a blending of the scheduler with a cpu-freq governor:
A busy cpu is interpreted as a signal to increase the frequency, but if the frequency cannot be increased (due to big.LITTLE or a package power usage limitation, or something else I cannot envisage) but there are other cpus that have faster speeds still available then one (or more) processes can be selected to migrate cpus (but not necessarily all the processes from the slow cpu). (And I have not thought about how to decide to migrate back to the slower cpu either ...)
Obviously, this would not suit all workloads, but would provide a good starting point and seems like it could be of use in other asymmetric scenarios.
Sure, race-to-idle is possibly only applicable to current hardware (and maybe only batch jobs), but I also think that similar principles are likely to apply to user-perceived latencies during interactive tasks - especially if you can powerdown what is likely to be a power hungry core and keep the residual activity running on the slow core.