Currently you can use the cachepath directive to generate a classpath that refers to the ivy cache.
However, if you are generating a jar file manifest, this is not very useful.
Instead, you want to generate a classpath for a bunch of jars relative to the jar you are creating.
One way to make this easy would be to have ivy:retrieve have an option that causes it to also export a path similar to the way cachepath does. This path could then be used in the <jar> command to set the classpath on the jar file.
Alternatively, cachepath could be changed so that it accepts a retrieve pattern and it applies that pattern to the resolved dependencies instead of using the ivy cache paths.