After discussing further with HP Support, they will not support any custom ESXi image, even the ones they produce, unless if you have a support contract for the VMware ESXi hypervisor through HP. In our case, we purchase support for the ESXi hypervisor through VMware, and only get hardware support from HP on our ProLiant hardware, so in the case of using HP vs. VMware drivers, it's really up to us. This blog post from VMware discusses this very topic from their perspective:
http://blogs.vmware.com/tam/2012/08/a-couple-questions-on-vmware-vsphere-5-image-customizations.html
I'm not sure which route you're going. I'm on the fence. Past experience has shown benefits to keeping the drivers and firmware in line with what HP documents. However per the blog post above, when getting support from VMware, they may ask you to reinstall their certified drivers over what HP documents you should use to match against the firmware. So if you have your support split between the two vendors, if there's ever a driver/firmware conflict, you're stuck between a rock and a hard place.
Not exactly related to your original question, but if you end up opening support tickets this is something to keep in mind.