It is pretty hard to find free continuous integration server hosting for .Net, unless your project is pretty important.
You can get some basic CI server hosting by using Appharbor.com, it'll build your application and run your unit tests using Gallio, it is not extremely configurable, but enough for small-medium projects.
I have set custom build targets in my projects, and Appharbor ran them correctly (failing the build if the target failed), the only small problem is that you get no beautified version of the results of custom targets, you will have to read the build log.
You can run custom targets as part of your default build process, which could include code analysis, documentation generation, etc.
As a little extra, you can also get a notification of your Appharbor build process in your desktop, using this .Net app: Appharbor Lookout