ESXi does not support Fake- or Chipsets RAID because of driver support.
If you boot from USB or similar you can try to Passtrough the Onboard controller so a VM including the drives and created a software raid their and use it as a Storage VM (export NFS/iSCSI).
I was afraid it is going that way.
Indeed your suggestion sounds like best option.
Perhaps i will use www.openmediavault.org (freeNAS maybe), or a convenient desktop linux distro for that....