You could add a new HTML, text or labrl field to display your warning. Add the field to the incident form and make the visibility rule based on the priority of the incident e.g. $(Priority == '1')
Now when the priority is set to 1 your new field/warning should display to the end user.
Thinking about it you could add a checkbox as well that the user has to tick in a P1 scenario to accept the fact it has been set to a P1 prior to saving, bit more work but depends on what your workflow does or who the P1 alerts get sent to.
That could be a good solution, thanks Alasdair.