I have the mini-CD for openSUSE 10.3. Would it be possible, by setting the repositories manually, to use this to install openSUSE 11.1?
I doubt it. It would have an old kernel and initrd. Just get the latest version, it’s only around 100MB and you’re going to download hundreds if not thousands of MB of packages later anyway.
It’s probably better to download the 11.1 mini cd or live version and do it from there.
You could try adding the 11.1 repos, but you may find you get more problems than just a simple upgrade.