Hello,
There is no way to span a port in a VXR. You could use PBR to redirect WAN traffic to an external router and then back to the VXR and put a switch in the middle for SPAN but that will most likely cause more trouble than it's worth. I would consider deploying a 6500 or 7600 with a flexwan module and use vacls to capture WAN interface traffic. Then use either an IDSM module or connect an external IDS to a GigE port to monitor the captured traffic.
Here is config example for the flexwan:
!
vlan access-map wan 100
match ip address 100
action forward capture
!
vlan filter wan interface serial 4/1/1
!
access list 100 permit tcp any any
!
interface gigabitEthernet 5/1
switchport capture
!
Good luck,
Jim