Here's my problem with your theory.

The word 'forgot'. It indicates that a pen exists somewhere.

You seem to be saying in your theory that no pen exists to transscribe the experiences of the mind. I agree with what you're saying, and I think it's a great theory and certainly very interesting and great to discuss, but in the context of the song I don't think it works.

If the line had been 'But I can't find a pen', I think it would work - but 'I forgot my pen' indicates that Maynard (or the writer) has a pen, has used it in the past and just doesn't have it in this instance.

Perhaps I'm being overly analytical, but that one word seems to break down your theory for me.