As for as I know, there could be some ways for your idea with MyBB 1.8, as follows:
- Edit the template of a theme to mark some checkboxes
checked
and display the text/etc. under those checkboxes by default. Doing this will affect all users using that theme. They'll be displayed with the same pages as you do when they're using warning system and set this theme as their default.
- Or, find or write a plugin that will attach a function do the job for you to the
warnings_do_warn_start
hook. warnings_do_warn_end
to send an anonymous PM to the warned user. But there might be a lot more work to do to check if a PM is already sent.
- Or, you might seek some extensions to your browser with the "auto form fill" feature.
For future MyBB 1.9 using the Twig template engine, your request would be fulfilled more conveniently, I suppose.