I don't really think it was ever a serious plan, or if it was it would have been a Children in Need-style spoof. For one thing, RTD's era had already acknowledged Trek as being a fictional franchise -- anyone remember Rose calling the Doctor "Mr Spock" in The Empty Child?
That was in
Series One, and this was a plan that RTD had initially had for Series One.
And I for one don't see why both shows couldn't have done a crossover story that was out-of-continuity for both series.