Launch Eclipse using Terminal: /bin/bash -ic /Applications/eclipse/Eclipse.app/Contents/MacOS/eclipse (your path may be different)
Add <property environment="env" /> to your build.xml and use the OS X environment variables that were set when Eclipse was launched however you want!
For instance, ${env.GEM_HOME} will give you the directory to the gemset that was loaded when Eclipse was launched, which might be useful if you're using RVM to manage Ruby binaries and per-project gemsets:
<apply executable="${env.GEM_HOME}/sass" dest="../css" verbose="true" force="true" failonerror="true">
...
</apply>