The time lock basically took the participants, area and times of the war outside the universe's timeline. From the point of view of the rest of the universe outside the time lock, the war never happened with corresponding retroactive changes to history, e.g. The Nestene consciousness's homeworld was destroyed.
The war being outside the normal time stream, time travellers cannot jump to before the war to take part as there is, in effect, no war within the boundaries of the universe to jump before.
There are a couple of notable exceptions to this: Dalek Caan rescues Davros by jumping inside the time lock briefly (and possibly inadvertently), at the cost of his own sanity. The Time Lords also planned to escape, but would have resulted in the destruction of the universe by bringing the rest of the Time War with them. This required a convoluted plan retroactively modifying the Master's memories and using a "whitepoint star" as a link.