You might try something like this:
DiscoursePluginRegistry.serialized_current_user_fields << 'recent_time_read'
to add it to the serialized current user.
User.current().recent_time_read
Keep in mind that this will only be serialized once, on app load, and that to get an updated value you’ll either need to query for it directly, or use MessageBus to ‘live update’ the user’s read_time.