We have an opportunity page layout that has a custom button on it for feedback regarding that opportunity. If the opp stage ='won' we want to force the user to provide the feedback. This can be done by a validation rule easily enough. However the page layout with the error message that effectively says 'please provide the feedback' does not show the custom button, it just shows the Save & Cancel buttons. Can the custom button be put on this error message page as well so the user can click on it & provide the feedback and save?
You can't do this using standard pages in a supported way I'm afraid. Custom buttons don't show up on the edit page, and that page is pretty much uncustomizable apart from the fields and sections.
You could put a feedback field on the opportunity and make the user fill that in when they edit an opportunity and change the stage to 'won', but you'd probably need another validation rule to stop people filling that field in when they shouldn't. Its not a great user experience but its the best that I've been able to achieve without resorting to code.
If you need to change the standard edit functionality, you'll need to go the Visualforce route I'm afraid.