Thanks for the input Walter!
I think I recall someone saying at some point that their company runs a custom LaF built off JTattoo... so perhaps it's not that hard. Looking at Substance though, it's quite the codebase... not sure if we'd be able to make JTattoo do some of the same things without major changes.
I took a gander for other possible LaF libs, and there aren't really any others that are good or better than what we've got (besides those nice synthetica ones they won't let us have!).
This fall I was going to try to organize a team at my University to attempt a JavaFX port of Spark's GUI. This will be a massive undertaking and possibly can't be finished in a semester's time with volunteers, so no promises here (everything from spark's SwingWorker impelentation to JOptionPane popups, to just about all threading and GUI components would need to be ported to the JavaFX framework). However, the new graphics libs in JavaFX 2.2.x+ are pretty nice and I find more responsive feeling and more modern/native-system looking than Swing programs (just based on my personal experience). JavaFX is "themeable" from good 'ol CSS too... so creating intricate designs should be easier and more possible. JavaFX is becomeing/has become the next gen java GUI library, and so far I like what I see. Anyways, I digress.
For the here and now, I'd support some testing of the new (in)Substance library integration, and if it does not cut the mustard, then we yank it for the 2.7.0 release (or 2.7.1 release if we want to try it out in a release for a short while... ). The version in the patch includes over 86 closed tickets in github's tracker... many of which are bugfixes, resource usage fixes (mem leaks), and other tweaks.
Only problem is I"m unsure how to test the specific issues reported before... other than send myself some files on two instances of Spark and see if it works LOL. Integrating the new (in)Substance was reletively straightforward actually once I figured out Spark's LaF engine... no "hacks" were needed. I never integrated Substance into Spark before, so I'm not sure if my process this time around was easier than it has been before. The Substance jar file is a custom jar which bundles laf-widget and laf-plugin projects into the same jar (they are needed by Substance and all now part of the inSubstantial project).