Having worked with HP servers since G2 days I just thought I'd throw my two pence worth in.
I have done a lot of unusual things with HP servers for Dev and training purposes. A HP SAS crontroller (especially the P400-800 series) will work with any SAS or SATA disk type, it does not have to have any special HP firmware on. The array configuration tool will even let you config and monitor the disks too.
I have filled a G5 with OCZ solid state disks just to see how many VMs we could host in our training lab, I've also attached the MSA trays filled with disks off ebuyer to create a giant NAS for Backing up Dev environments and VM images.
This is not hacking, non of the HP firmware or code was changed, and as previously mentioned the controllers support these disks on stock HP firmware.
If someone has a load of old hardware they could make something useful out of then fair play, I found this as I now have a couple of trays of 2TB EMC disks that I was going to dispose of.
This would be unsupported by HP. This is not something that you would do with your live environment without accepting the significant risk (it may be ok for static data that you have a regular backup of, like a DML or Media server). It is also worth noting here that a second hand disk off eBay is probably worse than a new disk that has been purchased and put in a HP caddy as you don't know the history of the disk.