Doing this with real hardware is kinda risky and complex.
But what about virtual machines like VMware? They have a BIOS, but how can we get our hands on it?
As I’m using Mac OS X for all my daily work, I’m using VMware Fusion as a virtualization program.

The BIOS seems to be hidden inside the virtual machine monitor executable itself.
To make our lifes easier, we are going to extract only the x86_64 part out of the universal binary with the following command:

lipo /Libary/Application\ Support/VMware\ Fusion/vmware-vmx -thin x86_64 -output /Users/user/Desktop/vm64

Now, we have a Mach-O file with only one architecture, which is x86_64.
It doesn’t matter if you are dumping the BIOS out of the i386 or x86_64 version of the binary. It should be the same for both architectures.

After that, we can extract the appropriate section in the Mach-O file which contains the BIOS itself:

segedit /Users/user/Desktop/vm64 -extract __VMWARE bios440 /Users/user/Desktop/bios440.rom

And here, you go: bios440.rom contains your VMware BIOS.

After you modified it to your wishes, you can use a handy VMware feature which lets you specify a custom BIOS for a virtual machine inside its vmx-file.
Just add this line to the vmx-file of the machine you want a custom BIOS for and place the rom file in the proper folder:

bios440.filename = "bios.rom"

Happy BIOS modding!