I think the principles apply, but the specific practice may have to change a bit. If you look at the five-part "cluster immune system" I normally use, only one part is dependent on a large user base, and that is incremental deploy. You can still use all the automated testing, sandbox, continuous integration and real-time alerting practices just as well.
You might find that the smaller customer base is actually a big advantage, since it allows you personally get to know many more customers and therefore find it easier to find and debug problems in realtime. Just a thought.
For this we thought about recording user input, then replaying it on the new build as a pre-deploy step and looking for errors in logs etc. Obviously this would only work without major ui/schema changes. Have you tried anything like this?
I would suggest using something like Five Whys to figure out how different your situation really is, rather than assume you'll have a lot of problems due to a small user base: http://startuplessonslearned.blogspot.com/2008/11/five-whys....
You might find that the smaller customer base is actually a big advantage, since it allows you personally get to know many more customers and therefore find it easier to find and debug problems in realtime. Just a thought.