On Lettuce Cocoon 2.1.9:

Set up root sitemap and configuration files so that Cocoon will provide Xalan (default XSLT processor), Saxon 6 (as another option for XSLT 1), and Saxon 8 (the only currently-available processor for XSLT 2.0). These will be called "xslt", "saxon6" and "saxon8".

Status update: This has been set up with the exception of Saxon 6. We're using Xalan because we don't see nay advantage to switching to Saxon 6. All XSLT 1 calls go to Xalan, all XSLT 2 calls go to Saxon 8.