Ventoy is UEFI-bootable. When it boots on a UEFI system, and you select an OS images, the system is still going to only allow booting a UEFI-bootable OS image. Since Spinrite is not bootable under UEFI, the normal bootable SpinRite images won't work.
To get SpinRite working as a Ventoy-bootable OS image, what might work is to construct a special boot image for Ventoy following the pinned thread here on Booting SpinRite under UEFI. Basically, you are booting a Windows or Linux image as a host OS for a VM which then boots in BIOS mode the SpinRite bootable image.
The catch is that you will have to manually reconfigure the VM after booting the Ventoy thumbdrive to attach the underlying host PC's drives so that the VM will "see" them when it boots and runs SpinRite.
So, I don't see this as specifically a "Ventoy" issue, but rather a consequence of SpinRite not being able to boot under UEFI. And there is more bad news on that front.
All of the new Z890 and beyond system board makers (Asus, Giga, Asrock and MSI, plus OEM Intel) have dropped CSM.
After the Z790, the capability to run a 16-bit BIOS overlay has been removed, so it's UEFI all the way now for Intel system boards.
It will become more and more critical that either Steve figures out how to make SpinRite bootable under UEFI in a consumer/customer-friendly way, or SpinRite will be forever relegated to software that only runs on unsupported legacy systems. Perhaps packaging a UEFI Linux host with a VM and SpinRite image with easy to follow instructions for attaching the host PC drives to the VM could be a useful solution until SR can boot directly on a UEFI system.
That isn't to say SpinRite won't be useful or a valuable tool. What it means is that in the very near future, starting in 2025, the systems that can even run SpinRite will soon forever be a dwindling number, as there will not be any new system boards manufactured to support the legacy BIOS mode SpinRite requires.
I hope Steve carefully considers the dwindling window of opportunity to get going on the development of SR7 Pro and the capability to boot on UEFI systems. Because that is all there will be within a year or two at the most, with Intel dropping CSM requirements from its system boards. Whether AMD follows suit or not, who knows.