Thank you for the reply. That is a mistake I've made in the past, but not this time.
The 7.2 USB image I used is a file named install72.img, which is 696745984 bytes in size and has an sha256sum of 38d796fd50e22201273fda11b753fc66d542abb0a7e61bb39a 928d9c1fc7e8f, which, when I last checked, matched that of the download site. The file was downloaded from
http://www.openbsd.org/faq/faq4.html#Download.
The previous, 7.1, USB image is a file name install71.img, which is 696745984 bytes and has a sha256sum of 5aa75092893b4ed969797d2a1f78e3db589004a90847e5457e 5aeb43435d46d3. This file was gotten a while back, so I can't say for certain that it was downloaded from the same page, but odds are, it was.
Were it not for the sha256 check and the fact of it working in qemu, I would have assumed that I'd gotten a corrupt file. Most odd. I wonder, has anyone else successfully booted from the install72.img on non-emulated hardware?
note: The sha256sum for 7.2 was checked against:
https://cdn.openbsd.org/pub/OpenBSD/7.2/amd64/SHA256