Once, Prince’s use of “U” for “you” and “2” for “to” (or “too” or “two”) was seen as an example of his supposed eccentricity. Now, of course, it is all too commonplace.
What Prince was up to could be called “sensational spelling” though now it is not so sensational (and that sounds a little naff) with the rise of text speak. Naturally, http://onlinelibrary.wiley.com/doi/10.1111/1467-9481.00127/abstract.
One amusing 18th century example of this is Pot-8-Os. Here he is:
And here is his Wikipedia bio, which reveals him to have been an equine member of the 27 club. Here is one origin story for his name:
The origin of his name has several different versions. According to one, Abingdon intended to call the young colt “Potato” and instructed the stable boy to write the name on a feed bin. The stable boy facetiously spelled the name as “Potoooooooo” (Pot followed by 8 “o”s), which so amused Abingdon that he adopted the spelling
Séamus Sweeney 