For a brief period in the early 80s, KYST (920 AM) Texas City/Houston, TX ran an all-Beatles stunt format; during this time, they used the fictitious call sign "KBTL". They still, however, used their real calls for the top of the hour ID.
Also in Houston, in the late 90s when KKBQ 790 broke their simulcast with country KKBQ-FM 92.9 ("93Q Country") and launched an adult standards format, they informally identified throughout the hour as "Unforgettable 790 KBME", even though their legal call sign was still KKBQ, which was still used for their TOH ID. (Although, they did finally legally get their calls changed to KBME a few months later, which they still have to this day, even though they flipped to sports in 2004.)
As for fake W calls, there is the news-talk station in Upstate South Carolina which brands itself as "WORD", formerly on 106.3 FM, now on 98.9 FM. Their legal call sign is actually WYRD-FM. They used to simulcast on WORD 950 AM as well as WYRD 1330 AM, but those stations flipped to sports in 2014, keeping the news-talk format only on the FM signal. (The WORD-FM calls are actually in use by a Salem-owned Christian talk station in Pittsburgh, PA.)