How to load Clang plugins with clang_parseTranslationUnit (libclang)

I want to do static analysis using libclang from Python.
This involves non-standard C++ attributes like [[custom::myattr]] that I let Clang convert to [[clang::annotate("custom::myattr]")]] through the magic of a custom plugin with a ParsedAttrInfo.

All well and good so far, I can run

clang-15 -Xclang -ast-dump -fsyntax-only -Xclang -load -Xclang libMyPlugin.so test.cpp

On my code and I get for example -AnnotateAttr 0x562219792be8 <col:7, col:18> "custom::myattr" as part of the AST dump - perfect.

Now I want to use the Python bindings around libclang to analyze the AST, so I parse my file like this:

import clang.cindex

clang.cindex.conf.set_library_file("/usr/lib/llvm-15/lib/libclang.so")

index = clang.cindex.Index.create()
tu = index.parse("test.cpp",
    [
        "-std=c++2a",
        "-I/usr/lib/llvm-15/lib/clang/15.0.0/include",
        "-Xclang",
        "-load",
        "-Xclang",
        "libMyPlugin.so",
    ]
)

and I do not get the annotate-tokens. So I have to assume that the plugin is not loaded (passing a wrong path for the shared library also does not result in an error).

I found some older posts dating back to 2014 to 2016:

[cfe-dev] libclang and plugins (2014 on lists.llvm.org)
Loading plugins into libclang (2016 on discourse.llvm.org)
libclang plugin support (2016 on discourse.llvm.org)

but since these are over 5 years old now I am hoping that what I want to do is possible by now, maybe I am just passing the wrong arguments?