You can use pomegranate to load new libraries and add them to your classpath at runtime. It will not be read directly from your project.clj, but it does use compatible syntax (using the Aether library that Leiningen 2 uses for native dependency resolution).
The usage looks like this (quoting the README example):
=> (add-dependencies
:coordinates '[[incanter "1.2.3"]]
:repositories (merge cemerick.pomegranate.aether/maven-central
{"clojars" "http://clojars.org/repo"}))
, , .