The Hirens boot CD (12.0 latest) is grub4dos based and has some AV utilities built in. They could be updated and new ones added.
I'm obviously not getting the reason for this AV checking to be done in a live boot environment ?
For the MBR with a stable partition structure, the MRB should be a static 512byte binary. A bit check of 2 small binaries is probably the safest check against inserted assembly code.