I want to put one RAID on top of other RAIDs. This can easily be done with MDADM by using an existing MD device as one of the members for the new MD device. Similar functionality could probably be achieved by "putting" a PV on top of an LV, but LVM seems to discourage this and there seems to be a much more elegant solution with "SubLVs". This is even done with the RAID10 "level", but is seemingly not possible with arbitrary stacked RAID levels.
Ultimately I want to use multiple smaller hard drives with larger hard drives in the same RAID. Previously I did this with a combination of MDADM and LVM by using MD devices as PVs in the top-most LVM RAID, but I'm hoping there's a solution using only LVM.