What I'm wanting to do is somehow run BIOS-only OSes on computers that do not support BIOS booting or CSM. Does there exist an efi application that I can chainload that can act as some kind of emulator for legacy BIOS?
For my purposes, using a VM defeats the point. I'm looking for as close to bare-metal as possible.
A "BIOS emulator" would mostly be a full BIOS implementation, minus the motherboard-specific hardware initialization code. The one that's typically used in such situations is SeaBIOS, which is used e.g. on machines that use Coreboot firmware (dual-booting Chromebooks and such) in a very similar way as you're asking for UEFI.
SeaBIOS also seems to be available as an UEFI firmware package (i.e. it can be the CSM embedded in your firmware), though apparently not as a standalone .efi executable.
I am not sure, but I believe GRUB or REFIND (Or it's Mac-compatible fork ReFIT) can boot mbr, but it is really a function of the BIOS to load the legacy MBR/CSM (compatibility support mode). Your best bet is to install these tools and try to see if they will work on your hardware.
Edit: after doing some digging, the answer is likely no: https://www.freebasic.net/forum/viewtopic.php?t=27466 https://mail.coreboot.org/hyperkitty/list/seabios@seabios.org/thread/GNX7DJC7BF6JUSKFZVGSZEWWZG2FUPDR/
