Hi Stu,
You can easily add an Html editor control to your custom feature and then you can access the .Text property of it on postback to get access to the html entered by the user.
The easiest thing would be to look at the code for one or more of the existing features, ie look at the code for the existing Html content feature or the Contact Form. You can see example code there for declaring the editor in the .ascx and configuring it from code behind and accessing the html from the .Text property on the control
The Building a Guestbook video tutorial series also covers use of the editor control.
Yes, my dog mojo was the namesake of the project
Best,
Joe