Here are some recommendations I got on another forum:

May be custom made, to go out far enough in front for a full grown dog, or out at the side of you for a younger dog.

http://arcticwolf.eu/products/bikejor-converter.html

The brush bow attached to this scooter, could work for you, again you may need to get it custom made.

http://www.dax-scooter.com/pages/fra...eset_order.htm

When or if you want the dog running out in front of you you might want to change to a pulling harness to some thing similar below.

http://www.ruffwear.com/Omnijore-Joring-System