Take a look at the Build with Parameters plugin. I've used that before to do something like this. We had a site that insisted that each feature and bug fix had to be a different branch, so we defined a branch build that used the Build with Parameters plugin to specify the branch to build.
All you have to do is define a Subversion post-commit hook that fires off the build for the project when a branch it is on is created or modified. It's a fairly straight forward trigger.