ODR will work across the ISDN connection. Here's a good link on ODR:
http://www.cisco.com/warp/public/105/39.html
If you're worried about routing updates bringing up the backup link...a simpler solution may be configuring a routing protocol and denying the updates.
access-list 101 deny igrp 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255
access-list 101 permit ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255
!
dialer-list 1 list 101