Without detailed documentation on the PDP-8 design process, we cannot say for sure. I suspect that while they may have briefly considered it, it was never a serious prospect.
The PDP-8 is just the PDP-5 redesigned electronically. The PDP-5 was introduced in 1963 as an even-more-reduced version of a computer compared to the PDP-1 and PDP-4. The PDP-1/4 did ...
I'm going to say no.
The PDP-8 was chiefly designed for compatibility with the PDP-5, and this machine also had no hardware stack.
There is not enough room in the instruction space to add push and pop instructions either.