I understand the difficulties regarding a lack of typical camera methods. Its true there is no zCore equivalent currently, but it is in the works for future releases. What I can offer for now is an approach I've used in the past to mimic the cameras used internally by zcore.
See the link below for my MimicCams.cs script.
Attach it to an empty game object in the scene. When run, it will child two new camera objects to itself that represent the same position and frustum transforms of zcore's internal left and right cameras. If you're looking for a "center" eye, you can follow the same procedure demonstrated, but use ZCore.Eye.Center
I'm not sure I fully understand what you're trying to accomplish. This could be an "XY problem", so if the script doesn't accomplish what you want, feel free to explain at a higher level what your objective is, and I might offer alternative approaches.