depending on where the key is in the back, you could try and take off the seat with allen wrench as you remove the side plastic underneath and you might be able to either snake something trough and possibly pull a key trough. I installed a fender eliminator on my '03, but think had to be in the trunk to remove any of that. Best of luck!
Looking closely at the lock system, there aren't many options.
What comes to mind is using a coat hanger with a tight hook (maybe a small 90 degree bend would be best) on the end. Go under the seat through the front part of the opening on the left side of the tail where the helmet hook is. Try to catch the round part of the latch where the cable end is and pull towards you.