Remember there is a max speed of generally 25mph with chains or cables.
I'd be worried about messing up the wheel with the chain rubbing on it. Then I'd be worried about the chain or cables breaking and damaging the vehicle.
Yes, they help you turn since you are only doing 15-25mph. But like JCL said, if you only have them on rear, your front tires still aren't really doing much.

What mountains are you going to? You mentioned Tahoe didn't you? Maybe you could call ahead to a tire store in Sacramento to get a set of wheels/tires ready and you could swing by the store and get them installed on your way?
Tirerack should be able to deliver you a set of mounted/balanced winter tires/wheels with rush shipping if you need them this weekend.
My preferred choice for local stores is Discount/America's Tire Company. They ONLY do wheels/tires, unlike other 'Tire' stores that do everything.

Last option...BMW dealers should be able to get you a set of wheels/tires by Friday, but you'll be paying twice what you would pay at Tirerack or Discount Tire.

Given the timing, I'd probably choose a rental car also.

__________________
*********************
Olympic Peninsula, WA, USA
-----------------------------------------
2005 Mini Cooper 100k, British Racing Green/Silver-Black
04/2002 4.6is 140k, Silver/Black-Alcantara *FOR SALE*
1989 325iX 270k, Diamond Black/Black
1984 318i 370k *SOLD* (First Bimmer)
Honorable Mentions: '71 2002, '87 325, '90 325ic, '92 525it, '93 325i, '94 530i