Похоже, это не реализовано в JRuby. Под 1.9.2:
ruby-1.9.2-p180 :001 > require 'YAML'
=> true
ruby-1.9.2-p180 :002 > require 'YAML/syck'
=> true
Под JRuby 1.6.2
jruby-1.6.2 :001 > require 'YAML'
=> true
jruby-1.6.2 :002 > require 'YAML/syck'
LoadError: no such file to load -- YAML/syck
from org/jruby/RubyKernel.java:1038:in `require'
from (irb):2:in `evaluate'
from org/jruby/RubyKernel.java:1088:in `eval'
from /Users/matt/.rvm/rubies/jruby-1.6.2/lib/ruby/1.8/irb.rb:158:in `eval_input'
from /Users/matt/.rvm/rubies/jruby-1.6.2/lib/ruby/1.8/irb.rb:271:in `signal_status'
from /Users/matt/.rvm/rubies/jruby-1.6.2/lib/ruby/1.8/irb.rb:155:in `eval_input'
from org/jruby/RubyKernel.java:1419:in `loop'
from org/jruby/RubyKernel.java:1191:in `catch'
from /Users/matt/.rvm/rubies/jruby-1.6.2/lib/ruby/1.8/irb.rb:154:in `eval_input'
from /Users/matt/.rvm/rubies/jruby-1.6.2/lib/ruby/1.8/irb.rb:71:in `start'
from org/jruby/RubyKernel.java:1191:in `catch'
from /Users/matt/.rvm/rubies/jruby-1.6.2/lib/ruby/1.8/irb.rb:70:in `start'
from /Users/matt/.rvm/rubies/jruby-1.6.2/bin/irb:17:in `(root)'
Хотя я (пока) не эксперт по JRuby, в заметках о выпуске JRuby 1.6.0 сказано: compatibility with Ruby 1.9.2 to a very high level.
Возможно, это не распространяется на syck ..?