Sorry, something interrupted me. But I have a workaround until there is a proper fix:
You can edit the hidden site setting for who is allowed to use custom prompts in the Rails console (Using Hidden Site Settings).
SiteSetting.ai_helper_custom_prompts_allowed_groups = 10
10 is the ID of the trust level 0 group.
The setting is deprecated, but my impression is that the frontend still uses it to decide whether the custom prompt is shown.
custom_prompt is removed from the helper options if can_use_custom_prompts is false:
And this is based on
which is a setting that will be removed soon
I think the serializer might need an update to work based on the configuration of the custom prompt persona instead of the site setting.