Capture and alter a 404 from an internal server

[Pedro Mata-Mouros Fonseca]

Greetings,

I need HAProxy to show a costumized 404 page in case an internal host  
responds with it's vanilla 404. I cannot change the 404 page in the  
internal host and it's got to be HAProxy to do it. Using rsprep can I  
find and replace the 404 Not Found with a 302 and a Location:  
new_error_page.html header?

Is this the best/only way to do it?
Thanks so much.

Pedro.