Hi Steve,
You would have to know the module id of the Html content instance and then you could retrieve it using the Html content business object. Though I don't really recommend doing this because the Html feature will change over time and break your code. It is better if your custom feature can be completely self contained so that it exposes its own editor and manages its own content.
There is a ModuleWrapper control in mojoPortal that you "could" put on your custom page and set its ModuleID to the moduleid of the html and it will load the html feature on your page. I can't promise it will work exactly as you want but you might take a look at it.
Hope that helps,
Joe