I am not sure what type of BC you are using, but if your BC has D-rings on the shoulder straps, you could connect your light with a bolt-snap, and secure it to the strap with bike tube or bungee. I have included a few links that will show you what I am talking about.
How to Attach a Snap and
http://www.gue.com/files/page_images...tor_config.jpg
Your other option is to look for an accessory made by Cetacea. They specialize in accessories for divers. You might be able to find a retractor or lanyard that you could attach your light with.
Clips, Lanyards and Retractors