Flow-based synchronous routing

A client has two last-mile links to a CE router for redundancy. These links are currently running in active-passive configuration via BGP by simply;

• Advertising a default route from the PE router towards the CE over both links.
• The CE then applies an AS Prepend on inbound and outbound directions for the backup (passive) link to become least-preferred.

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What concepts can I apply to achieve synchronous traffic routing between the client to the hosted application in our environment over the currently passive link. While all other services are to remain routed over the default link.

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To technically achieve this my initial thought was to statically route the application server's range over the backup link from the CEs perspective. This allows traffic sourced from the LAN destined for the application to be sent over the backup link. However, I am not finding a solution to ensure that the return path, source being application and destination being the LAN, stays synchronous. This will follow the route advertised by the CE for the LAN range over the primary link. To combat this I thought of a policy-based route matching source of the application and setting the next hop to the secondary link's P2P IP. However, it is not possible to adjust the next-hop of a packet in the outbound direction. I cannot apply this policy-based route on the incoming interface of the application as it is on a different PE router and therefore can't reach the theoretical next-hop.

A diagram depicting the above.

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Otherwise my question is;

Is it possible to effectively route a "flow" of traffic (matching IPs/ports only) over one link?