Your view reminds me much of SGML profiles... perhaps they could be adopted, at the page level, to achieve the functionality that you describe? I'd balk at the overhead of merging that kind of information onto every page.
For instance, I could/might support a profile that defines site-specific regular expressions to be used to create XHTML elements or perform Creole-blessed parser functions.
Whether such a facility is near-term or long-term depends on reaching consensus about the goals for Creole 2.0 -- an activity that should be started immediately imo.
Is Creole 1.0 all there is ever going to be or is there now energy to start work on Creole 2.0? I don't yet see concrete interest for a new version from the authors of Creole 1.0. Although, to be fair, they're just returning home from Portugal
.