I have written a test for this patch and run it (it works correctly). I 
guess there is no reason why it should not be accepted now.
To recap, this patch add a void * data field to the pragma handler, 
allowing to pass extra data. If we want to use this field, we need to 
use the function c_register_pragma_with_data or 
c_register_pragma_with_expansion_and_data. The old 
c_register_pragma(_with_expansion) is kept compatible.

I give two diff and two ChangeLog, the first are for the patch itself, 
the second are for the test.

I have tried to make things as good as possible, if there is a remark, 
please, send me it. Especially, I am not sure about the format of my 
ChangeLog, if there is an issue, I am ready to change it.

Changelog gcc:

2011-06-08  Pierre Vittet  <piervit@pvittet.com>

         * c-pragma.h (pragma_handler_1arg, pragma_handler_2arg,
         gen_pragma_handler, internal_pragma_handler, c_register_pragma,
         c_register_pragma_with_data, c_register_pragma_with_expansion,
         c_register_pragma_with_expansion_and_data): allows to add data to a
         pragma handler using a new c_register. Old c_register keep old
         behaviour for compatibility.
         * c-pragma.c (registered_pragmas, c_register_pragma_1,
         c_register_pragma, c_register_pragma_with_data,
         c_register_pragma_with_expansion,
         c_register_pragma_with_expansion_and_data, c_invoke_pragma_handler,
         init_pragma): allows to add data to a pragma handler using a new
         c_register. Old registers keep old behaviour for compatibility.

Changelog testsuite

2011-06-08  Pierre Vittet  <piervit@pvittet.com>

         * g++.dg/plugin/pragma_plugin_with_data.c: New test.


Thanks!

Pierre Vittet