Allan,
A better approach would be to duplicate the existing HTML grammar and locate the “ embedded-code” rule. Here add another rule to include cheetah. The HTML grammar re-uses this “embedded-code” rule in most of the contexts where embedded code makes sense.
You will then have to select this (duplicated and modified) HTML grammar for your cheetah files, instead of selecting cheetah.
Would it not be better to create a “subclass” of the HTML language grammar and just add the “embedded-code” rule as well as an include of the original HTML grammar?
(But then I guess the HTML grammar will not change that much so it is not necessary to maintain easy updatability?)
Dan