Apple's API won't change how Siri universal search fundamentally works. Siri accepts user input and runs a query against a cloud-hosted index. Apple keeps a cloud-hosted index of their iTunes content. So do companies like HBO, Netflix, Hulu, and any other content providers who may wish to participate.
This won't work with locally stored content because neither Apple nor Plex nor anyone else maintains a cloud-hosted index of that content. Doing so would raise technical, legal, and privacy concerns. Even if someone did keep such a cloud-hosted index, they would then have to figure out how to let Siri search only for that specific user's content each time, which may not even be technically feasible.
What Apple should have done was to create a hook for Siri to query the user's local content (using their Spotlight index) at the same time it queries the cloud. This would have provided the true universal search feature everyone actually wants without the need for a cloud-hosted record of everyone's locally stored media.
As it stands, I don't think anyone is going to be able to offer Siri/universal search for local content. And for the sake of argument, anyone who did would likely face scrutiny over the privacy implications, particularly in light of the imminent passage of CISA. Apple dropped the ball on this, perhaps intentionally as a way to persuade more people to buy/rent/store media online instead of hosting ripped or pirated content locally.
Personally, I will not spend a dime on DRM-locked content. My latest lesson learned was the realization that the first season of The Newsroom I purchased on iTunes a few years ago does not work on Plex because of its DRM protection. So as usual, the paying customer is punished while the pirate is rewarded.