[TxMt] Extending Markdown bundle (newbie)?

[Ramon M. Felciano @ Yahoo]

Hi --

I'm trying to develop a new language syntax for project management, note 
taking and task management around GTD. I've been using Markdown but 
would like to integrate some of the task features of the GTD2 bundle as 
well as some project reporting scripts. Rather than develop this as a 
completely distinct language, I'd like to simply extend the Markdown 
bundle, based on special casing more specific syntactic matches of 
existing Markdown syntax. For example, where Markdown recognizes:

 - my colleague

as an unordered list, I'd like to extend it to recognize:

 - @call my colleague

as a todo item, with a phone call context. This allows specialized todo 
items to be spread throughout a project document, then extracted, rolled 
up, etc using reporting scripts.

This is my first attempt at bundle development, so I'm trying to figure 
the cleanest way to do this type of language extension. I've read the 
various docs and blog entries on language syntax and contexts, as well 
as http://wiki.macromates.com/Main/Howtos#bundleExtension, which is on 
target but lacks some detail. In particular:

1. How do you override or specialize certain syntactic patterns from a 
base language? The language docs show how to use "include" to reference 
a whole separate grammar, but I think what I'm looking for is to 
override only a single grammar rule. In the above example, should I copy 
the "unordered list" pattern from the Markdown plist file and use a 
modified version in the new language file? Or is there a way to 
"subclass" the syntax w/out this copy-paste model by including only the 
one rule?
 
2. Are there restrictions on the file suffixes for this new language? 
Should the new language share the same file suffix  as the original, or 
come up with a different one?

Any pointers to sample code or even bundles that already do this type of 
extension would be greatly appreciated.

Thanks!

Ramon