I've heard that excuse used before, that Sherlock is already a fictional character in the Whoniverse, therefore a Doctor Who/Sherlock crossover would never happen - however, that's not quite true if you think about it.
The Sherlock of old IS a fictional character - but it could be argued the Sherlock played by Benedict Cumberbatch is not really that Sherlock. It would be quite easy for a few quick throw away remarks from the Doctor about the coincidence of his name, his job and Watson (with Sherlock replying "I know, I know...you couldn't make it up, huh?") and then on with the plot. It's unlikely but this is Doctor Who and anything is possible.