In a nutshell, I don't know what a SHPC is (nor OSHP), so I'm looking for an additional Ack.
No problem, I'll get an Ack :) Meanwhile - here's a summary, as far as I understand it.
Originally PCI SIG only defined the electrical and mechanical requirements from hotplug, no standard software interface. So it needed ACPI to drive device-specific registers to actually do hotplug. At some point PCISIG defined standard interfaces for PCI hotplug. There are two of them: standard hot plug controller (SHPC) for PCI and PCIE hotplug for Express.
Now an OS can have a standard driver and use it to activate hotplug functionality. This is OS hotplug (OSHP).
So presumably this will work on targets that don't have ACPI? Assuming a competent guest OS of course. Have you tested this?