It must be this line in mame.c:
ui_display_startup_screens(!settingsloaded && !options.skip_disclaimer, !options.skip_warnings, !options.skip_gameinfo);
Unfortunatelly I have no idea of C, so I don't know how to edit it. If I remove the '!' I skip the OK screens but I have a screen showing the game information, which promts you to press the select key. It's a step forward, but not the real solution.