Cisco WAN :: ASA 5520 - Source IP Based Load Sharing

A customer has an ASA5520 and 2 ISP routers with one WAN link each, and wants to split the load over both routers based on source IP ("natted" IP on ASA). I found this excellent doc on the topic: {URL}. Using PBR to achieve this is an option I was looking at, but I have come across a possible loop doing this with 2 routers. Setup:

           -----------CE-1---------ISP-1
           |           |
           |           |
ASA55020----      HSRP |
           |           |
           |           |
           -----------CE-2---------ISP-2
 
Both Routers receive default routes via BGP, and customer networks are propagated via BGP as well (i.e. the customer can specify the return path for the traffic). The ASA5520 forwards traffic to a HSRP virtual IP for redundancy purposes. If one router or ISP fails, all the traffic should use the other router/ISP. The customer wants to specify which traffic is sent over which link, by defining nat rules on the ASA. e.g. traffic sourced from the network 10.10.1.0/24 will always use ISP 1, and traffic sourced from the network 10.10.2.0/24 will always use ISP 2.
  
My problem: if I use route-maps on both routers (CE-1 and CE-2), sending part of the traffic to the other, and one ISP link fails, in my opinion I have a loop, since part of the traffic will get sent back to the router it came from. Is there any other was to achieve my goal without using PBR? I have looked at CEF and GLBP, but I cannot seem to find a way to load share via source IP.