OK. I've attached an attempt of patch to ocamlfind that implements what 
you describe (a META file with just the directory field).
I think it does the right things, but I did not test it extensively.

Le 18/12/2015 22:43, Gabriel Scherer a écrit :
> There is no ocamldoc.cm <http://ocamldoc.cm>{,x}a in  ocamldoc 
> libraries today. We could probably change ocamldoc's build system to 
> build one, but for now an ocamldoc package would have no archive(...) 
> field, just the directory. It is not clear we need an ocamldoc.cm 
> <http://ocamldoc.cm>{,x}a archive if all plugins are to be passed to 
> ocamldoc{,.opt} which is already linked with ocamldoc modules.
>
> On Fri, Dec 18, 2015 at 8:14 PM, Armaël Guéneau 
> <armael.gueneau@ens-lyon.fr <mailto:armael.gueneau@ens-lyon.fr>> wrote:
>
>     Ok, thanks for the explanations.
>
>     As for the ocamldoc package, would a META file along these lines
>     do the trick?
>
>     # Specification for the "ocamldoc" library
>     requires = "compiler-libs"
>     version = "[distributed with Ocaml]"
>     description = "ocamldoc support library"
>     directory= "^ocamldoc"
>     archive(byte) = "ocamldoc.cma"
>     archive(native) = "ocamldoc.cmxa"
>
>     Le 18/12/2015 18:06, Gabriel Scherer a écrit :
>     > So I just tried¹, and the failure is as follows: > > $ ocamlbuild -tag use_ocamldoc test.cmxs > +
>     /home/gasche/.opam/4.02.3/bin/ocamlopt.opt -shared -I +ocamldoc
>     ocamldoc.cmxa test.cmx -o test.cmxs > File "_none_", line 1: >
>     Error: Cannot find file ocamldoc.cmxa > > It looks like the tag
>     use_ocamldoc expects a file ocamldoc.cmxa to exist, providing
>     library support for ocamldoc plugins, and it does not seem to
>     exist -- it seems that the preferred way to use ocamldoc plugins
>     is to use "ocamldoc.opt -g foo.cmxs", with ocamldoc libraries
>     already linked inside ocamldoc.opt. So just using > >   ocamlbuild
>     -cflags -I,+ocamldoc test.cmxs > > works and seems to do what you
>     expect. > > It goes without saying that this is not a perfect
>     long-term solution, as it assumes that ocamldoc libraries are
>     distributed with the stdlib. It would be cleaner to have an
>     ocamldoc package for them (just like ocamlbuild has a dummy
>     ocamlfind package indicating the location of its library files),
>     but it seems that this is not currently the case. It would be nice
>     (but not super-high priority) to have such a dummy package in
>     ocamlfind in the future, so that you can just do > >   ocamlbuild
>     -use-ocamlfind -package ocamldoc test.cmxs > > ¹: The most
>     time-consuming part was finding the code of a ocamldoc plugin to
>     compile, as the documentation examples are incomplete. Next time,
>     feel free to provide a tarball with a minimal example to
>     reproduce. > > On Fri, Dec 18, 2015 at 5:14 PM, Armaël Guéneau
>     <armael.gueneau@ens-lyon.fr> <mailto:armael.gueneau@ens-lyon.fr>
>     wrote: > >     Hi list, > >     I'm wondering how one should
>     compile ocamldoc custom generators using >     ocamlbuild. > >    
>     First, the ocamlbuild manual >    
>     (https://github.com/gasche/manual-ocamlbuild/blob/master/manual.md#basic-tags-)
>     >     says that the [use_ocamldoc] tag is deprecated; and that a
>     corresponding >     ocamlfind package should be used instead. Is
>     there such a package for ocamldoc? >     I did not found one. >
>     >     Then, I did not manage to build the plugin into .cma/.cmxs
>     files directly using >     ocamlbuild. My command line was
>     something along these lines: > >        ocamlbuild -use-ocamlfind
>     -tag use_ocamldoc -package compiler-libs plugin.cmxs > >     When
>     trying to build plugin.cma/plugin.cmxs, ocamlbuild complained that
>     "Cannot >     find file ocamldoc.cma/ocamldoc.cmxa"
>     (respectively). However, I can build >     plugin.cmo/plugin.cmx
>     using the same command line, then create the .cma/.cmxs >    
>     manually using ocamlc/ocamlopt - which I ended up doing. > >    
>     Is there a better way to do so? If the whole process can be
>     improved (how?), I >     would be happy to write the required
>     patches/documentation. > >     — Armaël > >
>
>
>