Since the egg has been found, here is the secret to unlocking the entire game:
The first clue is a variant on the secret in the book. Cut out letters, but the side of the cut out matters. Left cut-outs = maggot
Right cut-outs = brain
This is meant to lead you to the maggot brain album. There is a second clue there:
And finally do this to reach the secret area:
That gets you the copper key, but there is a hidden secret with the secret room:
This gives you everything. keys and cubes.
The easter egg is at the end of the final board. You will have to do some special jumping to reach it.
Here is a clue: forward, forward, up, up, double up & forward.
I really don't want to spoil the ending for you, so go get that egg. I promise you it will be worth it.
The secret full-unlock will be hidden somewhere else in the final game so this trick will only work in this version.