I have one setup like this where I definied more local storage domains (aka more mount points pointing to different disks with different speeds...)
I don't know if this can match your needs
I don't remember if you can configure local storage domain for a non-allinone setup.
If not, I think it would be interesting to offer it as an enahncement for future releases.
A possible workflow could be:
offer chance to add local storage only if the cluster is composed by only one node and in that case show a big warning that adding a local storage would prevent adding other nodes to this cluster therafter.
If trying then to add another node, show a message that it is not possible as it has a local storage domain attached