Does there exist a BIOS emulator for UEFI?

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.