I made one of those props that keeps the hatch ajar, but still locked.
I made it out of stainless flat bar, but and easier version can be
made from a small turnbuckle.

pics and links to plans here

http://shufti.wordpress.com/2011/06/08/vanagon-heckklappen-aufsteller-rear-hatch-propped-open-gizmo/

alistair