The default classes for unordered list in HTML do not contain tick. In order to use a custom list in HTML, you have to embed an image as a bullet.
On the other hand, the solution I earlier provided creates the bullets based on a symbol. This is one of the reasons why RadRichTextBox's HtmlFormatProvider does not support export of custom bullets and converts them. At this point, we are not considering implementing such feature.
Just an idea. Can't you use another format for intermediate document storage, like Xaml? This might be possible if you do not edit the Html in another editor. This way you will not have any troubles converting bullets back.
the Telerik team