You’re right, I forgot that possibility. In the case discussed in that thread, the hostname is not changed by the Transform Rule, but by another service called Cloudflare for SaaS. On top of that, a Transform Rule is used to pass (and modify, if needed) the input path to the final domain.
Notice that what was suggested was to prepend a path to the requested path by use of concat("/example", http.request.uri.path), but by using Cloudflare for SaaS the hostname would’ve also changed in the process.
I’m not familiar with Cloudflare for SaaS, but it may fit your use case, please check the documentation:
The Transform Rule limitation applies. However, the hostname is not handled by the transform rule, but by Custom Hostnames created on your account for each customer domain, under Cloudflare for SaaS.
As I said, I’m not familiar with how Cloudflare for SaaS works, but hopefully other members of this Community may be able to help you further. For that, you may prefer to open a separate topic explaining in more detail what setup you want to create, and what needs you have, besides having customer domains rewritten.