How can I set the classpath to a folder on my disk for a given JBoss application from within Eclipse?
You need to set the JBOSS_CLASSPATH system property. Read the JBoss manual along with the contents of run.bat for more instructions.
Also, if these classes are required by your application, consider putting them in the applications' classpath instead. Read the JBoss application deployment guide for more details.
In case of any further queries related to JBoss, it would be better if you post in the JBoss user forums to get quicker help.