Oh well, so much for my earlier optimism - it turns out, that for the time being at least, Sandcastle can't easily be used on a build server.
The problem is the Help 2.0 Compiler, which in a previous life as the Help 1.x Compiler, was shipped as a standalone product. However, Microsoft, in its infinite wisdom, decided to bundle the 2.0 version into the Visual Studio SDK; and of course, to install the Visual Studio SDK, you need to first have an installation of Visual Studio, which rather defies the point of a build server. Especially for small operations on a budget, having to acquire an extra VS license just to automate the creation of documentation is more than a pain in the neck, it's downright unsociable.
There are some guides on working round this, but they only cover up to VS2005 and its SDK, whereas we're using VS2008 SP1.
I'm going to have a play around with exporting chunks of my registry which has references to the 2.0 compiler, in the hope that that fixes things. I don't hold out much hope.