There was some discussion amongst the ZFS devs for such a feature.

As I recall it was to change the current mirrored read strategy to be aware of the speed of the underlying devices, and prefer the faster if it has capacity. Though perhaps a fixed pool property to always read from a given device was discussed, it's been a while so my memory is hazy.

The use-case was similar IIRC, where a customer wanted to combine local SSD with remote block device.

So, might come to ZFS.