> I've got room to spare.
> Any documentation on how to achieve this (or some pointers where to look)?

It should be documented in ovirt.org, and in RHV documentation:
https://access.redhat.com/documentation/en-us/red_hat_virtualization/4.4/html/administration_guide/

> I couldn't find LVM / block device under host devices / storage domain / etc and Google search returned irrelevant results.

I tested locally, LVM devices are not available in:
Compute > Hosts > {hostname} > Host Devices

Looks like libvirt does not support device mapper devices. You can try:
# virsh -r nodedev-list

To see supported devices. The list seems to match what oVirt displays
in the Host Devices tab.

So you only option it to attach the entire local device to the VM, either using
pci passthrough or as a scsi disk.

Nir