Does RbConfig::CONFIG["bindir"]
it have what you want?
$ java -cp Development/src/jruby/lib/jruby-complete.jar org.jruby.Main -e 'require "rbconfig"; p RbConfig::CONFIG["bindir"]'
"file:/Users/asari/Development/src/jruby/lib/jruby-complete.jar!/META-INF/jruby.home/bin"
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