The transformation of the capitalized tag names is done via the ConvertToXhtml filter. This is designed to eliminate side effects from inconsistent browser behavior where tag names are always returned capitalized under IE 7 and 8 by the innerHTML property of DOM elements.
Although, even if the filter is disabled, some browser may always convert tag names to lower case. Therefore, I am unable to suggest a way to use tag names with capital letters. You can try it on your end by setting the ContentFilters to None and test again the same scenario.
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