What's wrong with giving a really long low pulse to a 555 timer?
As I understand it, the output simply will not go high until the trigger goes high. If the timer times out before the trigger goes high, you will no longer have a uniform pulse-width.

This datasheet has a good description of the behavior: http://www.datasheetarchive.com/pdf-datasheets/Datasheets-25/DSA-494116.html