Right now the BSF3 code builds a single bsf3.jar containing both the API and engine impls, I think it would be good to be able to also have a separate jar just with the javax.scipt API classes. The engines could then go in one or more other jars, and there could be a bsf3-all.jar if you want everything in one jar. Its also easier (for me) to do this type of build stuff with Maven2 rather than Ant.
I've submitted another patch with these changes in: https://issues.apache.org/jira/browse/BSF-2 ...ant
