I did some research on this problem and got that freaking minimap working fine in World of Warcraft with Catalyst 9.5 on HD3870. The reason WINE do not export WGL_ARB_pbuffer is lack of GLX_SGIX_pbuffer in string returned by glXQueryExtensionsString() function. From what I've read this string should contain list of extensions that present in both client and server strings, and both client and server report this extension as available for me. It looks like driver bug to me, maybe minor one but leading to major ones in such programs like WINE.
If you want to check it by yourself comment this line
in dlls/winex11.drv/opengl.c and run some program that uses pbuffer functionality.
if ( (glxRequireVersion(3) && glxRequireExtension("GLX_SGIX_pbuffer")) || glxRequireExtension("GLX_ATI_render_texture"))
It is easy to write a check for this case using WineGLInfo.glxServerExtensions and WineGLInfo.glxClientExtensions but it would be ugly and won't be accepted by WINE developers.
Dear Mr. Bridgman, I know that you are not involved in fglrx development process but you can reach people who are. Please let them know that a lot of AMD customers would be very happy to see this long standing bug resolved.