either... find a small kid that can reach through the wee gap in the boot and shove him through and get him to pop the bonnet, then jump leads on the battery and you're in
or jack up the car and get to the alternator and or any other point of power, cabling close to the battery or something and same as before, hook it up and you're in.
it's a pain in the ass and we have had to do it a couple of times with my 520i but the trouble is my little brother is getting bigger these days and soon wont fit!
the boot should open regardless of power or not, allows you to get to the jack also.