Architecturally, it is definitely possible to interleave ads from an ad network into your server's http3 response stream by doing server-side ad network integration. Only the measurement needs to be client-side and directly integrated with ad network, if at all. The same underlying mechanism used for content security policy can be used for ensuring integrity of ad content even when not served directly by the ad network. So, any domain/url/path/extension/dom-id etc based ad blocking mechanism isn't going to work.
Very likely this will result in superior technical performance (latency and measurement), and superior ad performance (due to richer backend data integration and better ad selection). It is just a matter of time.