Two suggestions. I use both for Canon EOS 7d +100-400mm and they work very well. I haven't used them yet on a bike (too cold here still) but will.
Cotton Carrier. I love,love,love this. Saves my wrists from pain.Very secure with the strap around the front. Camera available immediately for action.
http://buy.cottoncarrier.com/product_p/600ccv.htm
Think Tank Digital Holster30
with the harness. I keep my set up permenantly in this case with the harness attached. You wear the camera on your front ie a front pack not a back pack ! ( It comes with a rain shield which I took off to accomodate the camera). It also works fine with a 1.4x converter attached.
Camera available almost immediatley for action.
Hope this helps
Regs
robin