TextField and TextFormat are native Player classes written in C++, not ActionScript. Adobe does not make this C++ code available. You can subclass both of them in ActionScript, and you should be able to make YourTextField use YourTextFormat. I'm not exactly clear on what you want to do with images, so I can't offer specific suggestions.
However, please be aware that these classes are being obsoleted in favor of the new Flash Text Engine ("FTE") classes in Player 10's flash.text.engine package. And an AS library called the Text Layout Framework ("TLF") builds on top of FTE to add capabilities that surpass what TextField can do, including support for inline images. Finally, Flex 4 components are built on top of FTE and TLF. (For an example of what FTE and TLF can do, take a look at the new Times Reader AIR app for the New York Times.)
Since FTE, TLF, and Flex 4 are the future of text in the Flash Player, you should think about whether you want to build an editor on them instead of on TextField, which is not being considered for any future improvements.
Adobe Flex SDK Team
Thanks for the information. I will definitely look into that. One question, I chose to go with Flex 3 because Flex 4 does not have an official release yet. I think I read that the time table puts the release in the fall of this year some time. I am very excited about what I have seen in Flex 4 and can't wait to use it in my development. Would you consider the current build of Gumbo (which from my understanding, is the only SDK that supports Flash 10 player) ready to be used in a production environment? This is important because I need to build out this app before the fall of this year.
On a side note; I am really glad to see the separation of skinning and component logic in Gumbo. In Flex 3 I resort to using the code-behind design pattern for visual component development. Keep up the good work!