However it is still possibe to add custom system prompts in the normal chat (premium, free and no accounts). This goes way beyond the current settings that you can set in your chatgpt settings. It intercepts into the request and adds the system prompt. You can find the tool here. Also need to say that this does not override the master system prompt but already changes the model completely.
I also opensourced it here, so you can have a look. https://github.com/jonathanyly/injectGPT