Display custom HTML output from plugin AND send output to AI

It should be possible to write a plugin that:

1. Displays the output in some custom widget to the user

2. Also sends the output in some format to the AI

This used to be possible with _CUSTOMOUTPUT, I don't know why it was removed. There is an important use case in showing the output to the user in some interactive way, and also telling the AI in some sense what that output was.