It turns out that about three years before the original question in this thread was posted, @unlocked2412 posted an item in the Tips and Tutorials section with nearly exactly the same title as this thread, "Return value from x-callback URLs in macOS", which provides the answer. I'm now using that as the answer to my my question, "Macro to Find a Bear Note with a RegEx?".
I, too, would like a native KBM solution because it would be conceptually "nicer" to not have to depend on a GitHub utility and it would save a couple of actions in my macros. However, based on many interactions with Peter throughout over four year's membership in this forum, I really doubt that it will happen any time soon. There is a straightforward working way to do this that has relatively little overhead. He has many other proplems that he is working on that will have a greater impact to many more KBM users. If martinfinke/xcall disappears and no one creates a replacement, that would substantially increase the likelihood that Peter would consider creating a native KBM solution. Until then, probably not. I can't speak for Peter, that's just my experienced guess.
I did get Peter to implement a native solution to a pet issue of mine. His implementation of the Space Changed Trigger made it possible for me to not have to create an AppleScript Observer to watch for NSWorkspace.activeSpaceDidChangeNotification. The AppleScript solution was complicated and somewhat unreliable. Peter's Space Changed Trigger, introduced in KBM v11.0, just works. Replacing xcall is not (yet) the same kind of issue.