How do I flash my RAM's SPD to make it work with a MacBook Pro?

I solved it!

I read up about other SPD flashing attempts and found out about a "Checksum correct" function, which is available in both applications mentioned in the question. I applied the correction and rebooted. No go, it seemed.

But then I remembered something about an SMC (System Management Controller) reset that you could perform if the system was behaving weirdly related to hardware functions. I gave it a go. The procedure for a MacBook Pro, later than Early 2009, is the following:

1. Shut down the computer.
2. Plug the MagSafe adapter in if it's not connected
3. On the built-in keyboard, press (left side) shift-control-option and the power button at the same time.
4. Release all buttons at the same time after the chime.

After OSX had finished booted, I went to check in "About this Mac":

Mission complete! I hope this will be of use for someone else wanting to unlock XMP profiles in their MacBook Pro.