Have a look at Screwfix.co.uk - they do some really small 12v batteries for under a tenner that can be charged with an Optimate and should run a 2610 for ages, you will need a new lead or BMW type socket but with a little imagination...
If you send TyroneM a pm he has made a very nice stand alone power pack that the 2610 clamps onto and uses rechargable batteries. He has got 5-6 hrs use from the batteries so not bad
He bought it along to foxes diner last sunday when we were planning a trip to Finland.
I thought I had found the perfect GPS, but like you I would like it to work off the bike.
I am still looking for a GPS comparable to the 2610, however if you work out a solution please post as I would be interested in purchasing one, then a 2610
I assume that your Garmin came with the cigarette lighter socking and the 3-pin power converter? I set mine up at hone then use it on the bike or in car. Other than that, any 12-v battery and some home-made wiring will get you a mobile unit.