Two possible solutions:
1. Have a WikiSyntax Object (A simple class with one property: a
combox box with different syntaxes: XWiki Legacy, Creole, MediaWiki,
Confluence, JSPWiki, etc) that users can attach to pages to tell the
Renderers what syntax is used. If no such object is attached then
it'll default to XWiki's default syntax (XWiki Legacy or Creole for
example).
2. Have some special syntax, independent of the wiki syntaxes to
tell the Rendered that such block of content should be rendered with
that given syntax. Again there would be a default.
3) A new "syntax" document meta ? WDYT ?