Er... has the "50th Anniversary Special" even been written yet?
Last I heard, Moffat mentioned "having a few ideas", but nothing much more concrete than that.
At this stage we've still no idea whether the anniversary will be celebrated in a single "special" episode, or whether the whole next series will be dedicated to it in some fashion. And if there is a crossover that brings together characters from the past with characters from the present, we don't have a clue if it will occur at the very beginning of the next series, or perhaps on the actual anniversary date of the first Hartnel episode, or if they'll build up to it over the course of the entire next series, with the crossover event taking place in the climactic final episode(s) of the season.
With the official anniversary as yet unwritten, and nearly a year in the future (give or take), isn't it maybe just a tad early for TPTB to be asking any
old cast-mates about reprising their old characters?
The point is: Just because Billie Piper has yet to be approached doesn't negate the possibility that she'll be approached tomorrow, or next week, or next month, or any other time between now and the time the "special" actually goes before the cameras.