You might try a mountain bike or lightweight iceclimbing boot with a lightweight randed gaiter.

Lightweight ice climbing boots are real expensive but if you happen to have large or small feet you can get them for a steal on closeouts from sierra trading post or places like that.

The randed gaiter can be glued to the rubber of your boots sole so it's waterproof and they usually aren't too bulky.

http://gearx.com/product_info.php?products_id=4519

These are the lightest and lowest priced mixed climbing boots that I know of. They have a very stiff sole thats great for biking.

http://www.mtntools.com/cat/alpineic...taculboots.htm