Interesting info! I've never been down in the utilidors and even though I know they are boring as heck, I've always wanted to go down them just to say I've done it once. Alas, I've been too chicken thus far
I know there is an entrance on the side of the castle as well as in Tomorrowland. Unless they've completely changed it since I was there last. Those are the only two the actual locations off. I will say Disney does know how to make them really inconspicuous. The entrances are so nondescript most peoples' eyes are drawn right to the next thing on. But I suppose that is the way Disney wants it