I managed to compile my project! The problem was that I had created a lein hoplon-castra project, which will not compile with Boot2.
The solution was to;
remove all trace of Boot/Boot2, then re-install boot.sh;
follow the most basic Boot2 tutorial to create a Boot2-compatiable project;
from my original hoplon-castra project, I copied the hoplon file, and assets folder, into the Boot2 project.
This way, the build war command worked just fine, and my project is compiled.
Now... how do I install Apache Tomcat to Ubuntu Snappy Core, to host the darned thing?!