I've got a Toyo 45CF (carbon fiber) which is light and folds up nicely with a 135 or 150 lens in place. Also, probably the cheapest 4x5 on the market today. I can hike and carry it in a shoulder bag.

I think if you were really clever, you could turn your bike upside down and use it as a tripod, with 1/4" bolt welded to the bottom.