Hi Christophe ,
Sorry I cannot access the system , but you may try the below steps
1) lo_button = Create a button using CL_SALV_WD_FE_BUTTON
2) Set the Image source etc e.g. setting of lo_button->set_image_source( '')
3) LO_ALV_CFG_TABLE = Get reference to cl_salv_wd_config_table using the interface controller get_model method.
4) lo_btn_settings = Get reference to standard toolbar function using IF_SALV_WD_FUNCTION_SETTINGS~GET_FUNCTION_STD
5) Finally set the editor of standard function passing the button created in first step lobtn_settings->set_editor( lo_button )
Still if it doesn't work , another option could be , hiding the standard functions and creating a custom filter/settings button .
then following the above stepsexcept the 4th step which should be replaced by creating new function using IF_SALV_WD_FUNCTION_SETTINGS~CREATE_FUNCTION .
And then in the event handler you can call the standard cl_salv_wd_table methods.
Hope this helps . I would be able to post the exact code hints by tomorrow once I get to office.
Best Regards