I just came a across the word 「お巡りさん」。While 「お巡り」 is a word, 「巡り」 is not. Likewise, 「ご飯」 is a word, but while 「飯」is a word, the reading changes to the 訓読み (めし)。This means that the 「ご」in「ご飯」is not window-dressing 美化語. The「ご」in「ご飯」is a necessary part of the word. In my opinion, 「お茶」 is in limbo. I've never heard 「茶」 spoken without an honorific 「お」、but 「茶」alone is officially in my dictionary。
Over time, 「お」or「ご」can become a necessary part of some words, right? Is this a documented part of Japanese grammar? (reference links would be welcomed). What are a few more words that have an honorific prefix, 「お」or「ご」, as a necessary part?