Prisma SD-WAN - Active and Backup Data Centers - BGP Return Path?

I'm looking to see if anyone else has run into this. I'm halfway through a Prisma SD-WAN deployment - almost 5,000 sites out of over 9,000. At this point, my company is considering deploying virtual IONs in various AWS regions around the globe and backhauling all traffic from those regions back to the US via CloudWAN. The issue I'm trying to solve for is how to handle the return traffic depending on which DC is active and which is backup.

Let's say that I have a site in Singapore. I want to have my Active DC be the local AWS region in Singapore and then my backup DC's be California and New York for instance. In the US for sites on the East coast, however, my Active would be NYC and backup California. If I'm on the West Coast, vice versa.

All of this is relatively easy to do in Prisma with Service and DC groups. No problems there. But that only affects the path FROM the site TO the DC. What about the return path? If I'm not able to influence BGP at the site level, how am I supposed to control the return path? I've posed the question to my account team and I haven't really gotten great answers. It almost seems like we'd have to have dedicated head-ends for each scenario and then prepend from the headend to the upstream BGP peer in the DC. This isn't looking too promising so far. And candidly this entire deployment has been a massive pain in the ass.

And no, the address blocks in each region are not contiguous so we can't build route-maps to prepend based on address space. I'd basically need to be able to prepend based on Domain.

Anybody else run into this or something similar or have suggestions?