Rebuilder,
I have no specific knowledge of the RAAF Canberra Mk21. However, the operational RAF T4 was fitted with Martin Baker Ejection Seats for all crew members.
I have some recall that the very early development aircraft may, only may, have used something different along the lines of what you describe.
The RAF Canberra T4 was assessed as being difficult to ingress, and it should be made impossible
A sentiment I wholeheartedly support.
lm