The pbuffer check is done in dlls/winex11.drv/opengl.c:
if ( (glxRequireVersion(3) && glxRequireExtension("GLX_SGIX_pbuffer")) || glxRequireExtension("GLX_ATI_render_texture"))
register_extension(&WGL_ARB_pbuffer);
Just make that if-statement return true or just remove the if-statement. That should do the job.
