Perhaps you may want to look into a product called Wilt-Pruf. Maybe this may have some extending effect when applied to the flowers. Altough with M. grandiflora and the other non-deciduous types, their tepals are pretty thick and waxy but shatter from the base pretty easily. The tepals are released in nearly pristine condition often times.

Perhaps this release is a response to being pollinated? What if one were to remove the stamens as soon as possible to prevent pollination? Or even the carpel? Would that extend the life of a magnolia flower?

I believe the easiest solution is to add another magnolia. :P Preferably 'Little Gem' magnolia, as this cultivar has one of the most extensive blooming season for M. grandiflora.