PVE 3.4 supports HP BL460c Gen9?

I do not have tested this pesonally but I see no reason why not. test it if you have such a hardware, and if you have issues report it here!
 
I was thinking specially around a NIC driver for Flexfabric Adapter. HP off course only specifies supporting RHEL +6.

Otherwise I assume I could install PVE on top of a RHEL 6 instead of the easy way from the Debian ISO image.

Will do once we get the HW...
 
Support for hardware always refers to the kernel and since HP claims support for RHEL 6+ and proxmox uses a RHEL 6+ kernel you are home free.