Here's something you can try from hackmac.org's article "Access Single-User Mode When Locked":
1) Change the Amount of RAM
Change the amount of RAM inside the computer. It doesn’t matter if you
add RAM or remove RAM, but the amount of RAM has to be different than
the amount present when the computer was last turned off.
Be careful when handling RAM. It is very sensitive to static
2) Reset the PRAM and NVRAM
Once you have successfully changed the amount of RAM, and preferably
closed the case back up, turn the computer on. Once you hear the chime
signaling that the computer is turning on hold CMD+Option+P+R. Hold
them down until the computer restarts and you hear the chime for a
second time. Immediately after you hear this second chime release the
keys, then quickly press them again. Wait until it restarts and you
hear a third chime. Then repeat for a fourth time. (Note: This is
resetting your computer’s PRAM and NVRAM)
3) Boot into Single-User Mode
Once you have heard the fourth chime hold CMD+S. This will boot you
into single-user mode. From here you can perform whatever actions you
want to do, but be careful not to change anything vital to the running
of the computer.
4) Replace/Remove RAM
Turn the computer off and put the original amount of RAM back inside.
This step isn’t necessary but is highly recommended.You should try to
leave the computer as close to its original state as possible.
Again remember that opening up a Mac does void its warranty.