Kaz,
I have an accessory socket on my 01 and 02 beemers that I use to trickle charge the batteries. I have never had to jump start my motorcycle, but would not use the accessory socket to do that.
I did a google search and found some information that might answer your question. I figured I should learn something about this as well. Good luck, Larry
The first two probabaly have your answer. The rest have good general information for others.
http://www.beemershop.com/tech61.php
http://www.wunderlichamerica.com/motorcycle/2024568.html
http://www.motorcycleinfo.co.uk/index.cfm?fa=contentGeneric.nvxszdkebklxuaer&pageId=717751
http://w6rec.com/duane/bmw/battery/index.htm
http://bmwsporttouring.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=269171
http://www.bmwmotorcyclebattery.com/difficult-to-access
Here's a link to this forum that has some valuable information as well.
http://www.crbmw.com/content/view/42/53