During the first launch of the Lancelot, the development team says that they haven't been able to install an ejection mechanism in it. Even though the Lancelot gets new upgrades as the story progresses, no mention about the ejection mechanism gets made. Did they finally install it?
It's assumed so since the page for the Lancelot Conquista, the upgraded version of the original Lancelot, lists it as one of its design features while the Lancelot does not list it. So one could assume that before it was officially ordained as the Conquista the Cockpit Ejection System was added in.
When the Lancelot was equipped with the Float System in Stage 20 - Battle for Kyushu it was designated as the Z-01/A (the Lancelot is designated originally as Z-01 while the Conquista is Z-01/D). C.C's Lancelot Frontier (which is more or less another Lancelot) is listed as having the Cockpit Ejection System and it too is designated as the Z-01/A1 so to narrow down the time we could assume that it was between Stage 20 the Ejection System was installed and the ordination of the Conquista
We know that the Lancelot Albion had to have had one installed regardless for the successful completion of the Zero Requiem
As Suzaku needed someway to escape from his exploding Knightmare when he faked his death to later assume the role as Zero
Also on the Cockpit Ejection System does say this
All production-model and most prototype Knightmare Frames are equipped with this system. The only Knightmare confirmed to lack a cockpit ejection system is the Z-01 Lancelot. This flaw is later addressed in the Lancelot Frontier, which except for its color is otherwise identical to the Lancelot's. It is unknown whether Suzaku's final machine, the Lancelot Albion, had an ejection system.
though the last sentence seems contradictory since the Lancelot Albion's page says
In the last episode, it is hinted that a cockpit ejection system might have been built into it.
1 - the designantions are shared between Knightmares when massproduced. the Glasgow's designation is RPI-11 which is shared by the Knightpolice (who are retired Glasgow's) so the Z-01/A being shared between the Lancelot and Lancelot Frontier could be an indication that the Frontier's specs are what was planned for possible Mass Production of the Lancelot before the inception of the RPI-212 Vincent which later led to the Mass Production Model RPI-212B Vincent Ward