After much googling, I haven't worked out where all of the JVM arguments are set when JBoss is started. I did notice upon server startup this message:

JAVA_OPTS:

Which was followed by what appeared to be the JVM arguments. So I opened up bin/run.sh and sure enough, a string is built up containing various JVM arguments. I simply hacked the required JVM arguments onto the end of the string to make everything work.

Look for the commented line:

# Setup JBoss specific properties

And edit the JAVA_OPTS variable below so that it includes:

-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005

The next time you start JBoss, you'll be able to perform ColdFusion debugging from Eclipse!