Cards do not always take precedence. The "Can't beats Can" rule would still not allow you to change the flavor text. That's why cards like Ætherflame Wall say "can block creatures with shadow as though they didn't have shadow." instead of "can block creatures with shadow". But regardless, you can't change the flavor text to "you may change this text as though that text was text from a Magic: The Gathering card" in the first place. If you could change the flavor text to that, then yes, you can subsequently change the text to whatever. But that first requires being able to change ______'s flavor text into non-"mtg flavor text" first.
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So even with R&D's lair, this combo doesn't work.
According to Mark Rosewater's FAQTIWDAWCC, the most official source for Un- rulings, you can only change ______'s name to names that exist in MTG (You can't turn a Wild Mongrel aquamarine because you can only change items into things that exist in MTG) Likewise, you can only change _____'s Flavor Text into flavor text that exists for any mtg card. Link here: http://www.wizards.com/magic/tcg/article.aspx?x=magic/faq/unhinged