(2 hours pass)
Success. Following Charles's instructions here, I got JRuby, Rails, and a Rails scaffold up and running without major incident. Streamlined, however, balked at the code generation phase. I posted a question on the jruby-user list, and within minutes had the answer from Charles: the following syntax is not OK in JRuby, and apparently will be removed from Ruby 2.0 as well:
A quick regexp replace to this form did the trick:
[Note: Typo mentioned in the comments has been fixed. Thanks Jim.]
Streamlined (trunk or 0.0.6) now runs on JRuby on Rails. Bring on those funky enterprise databases!