The boot light is permantely live so you could have used this but seen as you can't get into the boot that won't help! You can use the heavy duty red wire which goes to the solenoid/starter as said and a ground, i.e. the engine earth strap, that as chip suggests this will be enough to at least power up the alarm system and thus you can then open the doors.

I too have been in the same situation as you are in and I managed to open my bonnet from the outside, it wasen't easy however! The method you have is a better option I should think.

Martin