There's not a lot I can do about this. Cap'n Proto adoption doesn't directly drive revenue for anyone in particular, so I can't hire an army of engineers to throw at it... People who want better Cap'n Proto support in each language need to step up to help make it happen.
One thing I am looking at doing is making it easier for per-language serialization implementations to bind to the C++ RPC implementation. This might make a lot of sense, since the serialization implementations have wide APIs but shallow implementation details, while the RPC implementation is a pretty narrow API with very complex implementation. And it turns out Cap'n Proto messages are super-easy to pass between languages since the in-memory format is by design the same across languages -- passing around byte buffers tends to be pretty easy.
Depending on how much more mature the C++ implementation is, you might consider using the Rust version for this instead. I've toyed around with exposing Rust to C (for a TCP-based message protocol[0], as chance would have it), and it worked pretty great.
1) You end up with APIs that are not idiomatic for the calling language. For instance, D supports properties, where C++ uses separate getters and setters. Also, FFI wrappers tend to add an additional layer of ugliness in order to translate features that don't exist in the calling language. If it were an API you only used in one small part of your code maybe this would be fine, but spread all over your codebase would be awful.
2) The generated getters and setters are designed to be inlined for best performance, but cross-language inlining is often not possible. In fact, most FFI wrappers incur a runtime performance penalty to convert between different conventions, and this penalty is going to be extra-severe when calling functions that are intended to be lightweight.
So this is why I say that the serialization layer -- which includes all this generated code that apps interact with directly -- should be native to the language.
But, you could use the native serialization layer to construct messages, and then pass it off to the C++ RPC implementation. The RPC implementation has a fairly narrow API surface with an extremely complex implementation behind it, so it's a perfect candidate for this.
Currently system is in Java+JS front end. I feel that JSON serialization that I currently use, is not the right thing..
But at the same time, I care about 'size in kb' of the js front end. Therefore have been learning the options.
That to be said, it's really ugly as an API.
Why I'm asking for this - for the simple reason - I don't want to deal with port allocation on a CI.
In fact, you can adapt the RPC system to operate over any kind of byte stream transport pretty easily, by implementing the kj::AsyncIoStream interface. Or if you already have a standard file descriptor (or iocp-compatible HANDLE in Windows), you can use that.
One fancier thing that's still on the roadmap is shared-memory IPC. Cap'n Proto's zero-copy serialization was really built for this, but so far for all my real-world projects, Unix sockets have been fast enough, so I haven't been forced to full implement a shared memory transport yet. Maybe soon?