> ocamlfind ocamlc -package sexplib.statements -syntax camlp4o -c data.ml

To me it seems the only difference here with the case without camlp4 is 
the "-syntax camlp4o" option. If you add something like:

After_rules ->
   flag ["ocaml"; "pkg_sexplib.statements"] (S[A"-syntax"; A"-camlp4o"]);

to your myocamlbuild.ml, and tag your files with 
"pkg_sexplib.statements", and use the plugin I wrote which declares the 
"pkg_*" tags, does it work? (ocamldep might still not work so you might 
need to tweak it a bit with tags "compile", "link" and "doc").

Anyway, I don't think you need to modify Ocamlbuild to make it work 
(unless you don't want to use a plugin) if it is just a matter of adding 
"-syntax camlp4o" at the right moment. For the "-package" option, my 
previous solution should work.

That being said, once Ocamlbuild handles multiple plugins, it will be 
easy to write your own, standard, generic plugin in your .ocamlbuild 
directory and use it in all of your projects :)

-- 
Romain Bardou