Is it possible to...

[Benrevs]

In the past I have just touched a length of wire from the battery+ to the solenoid on the starter an voila!  You will need the ignition on though.

[The Mighty Elvi]

Tie a piece of string to the top of the key. Loop the string over the top of the key behind the back of the key then underneath the steering wheel, through the sunroof and into the engine bay.  Give it a tug and vroom vroom.

[Diamond Hell]

Ignition on and then spanner across the starter terminals anyone? 

[russ-vdub]

Tie a piece of string to the top of the key. Loop the string over the top of the key behind the back of the key then underneath the steering wheel, through the sunroof and into the engine bay.  Give it a tug and vroom vroom.

Hahaha!