Hi there, welcome to Charmhub 
The charm you reference there is using a slightly outdated pattern (though it’ll continue to work!). The only pain point here is that there is not much active work to add more supported fields to our own pod_spec definition.
Meanwhile, we have introduced a slightly different method of managing workloads on Kubernetes outlined in this post.
There are several differences here, and while support is early, we do enable easy access to the Kubernetes API, which would allow you to change that field retrospectively after Juju has created the pods. The full documentation for the Operator Framework (and examples of how to get started with the sidecar approach) is at Juju | The Charmed Operator Framework.
Additionally, I wrote a charm for the Kubernetes Dashboard that makes some more involved changes to the underlying deployment using the Kubernetes API which might be of use to you.
Feel free to share your code, or elements of it and we can try and get you started!
Thanks, Jon