I got NFS to work. So I can now use a shared upload directory across two remote machines (e.g. a link [[Image:file.jpg]] that isn't found locally but exists in the remote wiki directory will be properly displayed).
Transclusion, however, is still now displaying the images within an article. If I check the html source then I can see that the img scr attribute points to a relative path on the remote wiki that is not resolved by the browser (hence, it displays stub box with a hyperlink to the image article on the remote wiki).
So setting $wgUploadBaseUrl=$wgServer; would solve this problem again, but then I wouldn't need the whole NFS setup again.
Any ideas how to get transclusion with images to work without $wgUploadBaseUrl=$wgServer; ?
Btw, $wgUploadBaseUrl=$wgServer; causes a wrong path if the remote wiki installation is not in the top level directory (yoursite.com) but in a sub directory (yoursite.com/wiki).
And I still haven't figured out how to configure $wgForeignFileRepos and whether this would make any difference?
Thanks, Andi