I don't know whether is supported feature or just as a fallback, but valac puts annotation tag when generating GIR file from vala files.

But when g-ir-compiler read the GIR file, it does not know about "annotation" tag.
It seems that it expects "attribute" tag instead of it.

I tried generating gir from c source file with a free-form attribute.

some-vala.vala

> namespace SomeVala {
>     [Bar (baz="true")]
>     public static void foo () {
>         // Just do nothing
>     }
> }

SomeVala-0.0.gir from "valac some-vala.vala --gir=SomeVala-0.0.gir --library=somevala -C"

> <?xml version="1.0"?>
> <repository version="1.2" xmlns="http://www.gtk.org/introspection/core/1.0" xmlns:c="http://www.gtk.org/introspection/c/1.0" xmlns:glib="http://www.gtk.org/introspection/glib/1.0">
> <package name="somevala"/>
> <c:include name="some-vala.h"/>
> <namespace name="SomeVala" version="0.0" c:prefix="SomeVala">
>         <function name="foo" c:identifier="some_vala_foo">
>                 <annotation key="bar.baz" value="true"/>
>                 <return-value transfer-ownership="full">
>                         <type name="none"/>
>                 </return-value>
>         </function>
> </namespace>
> </repository>


some-c.c

> #include "some-c.h"
> 
> /**
>  * some_c_foo: (attributes bar.baz=1)
>  */
> void some_c_foo (void) {
>     /* Just do nothing */
> }

some-c.h

> #ifndef _SOME_FOO_H
> #define _SOME_FOO_H
> 
> void some_c_foo (void);
> 
> #endif


SomeC-0.0.gir from "g-ir-scanner --library=some-c --no-libtool some-c.[ch]  -o SomeC-0.0.gir --namespace=SomeC --nsversion=0.0"

> <?xml version="1.0"?>
> <!-- This file was automatically generated from C sources - DO NOT EDIT!
> To affect the contents of this file, edit the original C definitions,
> and/or use gtk-doc annotations.  -->
> <repository version="1.2"
>             xmlns="http://www.gtk.org/introspection/core/1.0"
>             xmlns:c="http://www.gtk.org/introspection/c/1.0"
>             xmlns:glib="http://www.gtk.org/introspection/glib/1.0">
>   <namespace name="SomeC"
>              version="0.0"
>              shared-library="libsome-c.so"
>              c:identifier-prefixes="SomeC"
>              c:symbol-prefixes="some_c">
>     <function name="foo" c:identifier="some_c_foo">
>       <attribute name="bar.baz" value="true"/>
>       <return-value transfer-ownership="none">
>         <type name="none" c:type="void"/>
>       </return-value>
>     </function>
>   </namespace>
> </repository>