SQL Server: multiple failover cluster instances with disjoint storage
I've found the answer to this problem. It is possible, but you either need Windows 2008R2 SP1 or higher, or a hotfix. The full details are at:

AlwaysOn Architecture Guide: Building a High Availability and Disaster Recovery Solution by Using Failover Cluster Instances and Availability Groups

To quote a line from this article: This arrangement of storage, where a cluster disk is shared between a subset of nodes within a WSFC, is referred to as asymmetric storage