Depends on how HTML-friendly the thing is supposed to be, and what it's for.
- If it is just, for instance to include some flash banner, or other 'semi-fixed' HTML you could and probably even should do that using your templates without need for a WYSIWYG at all.
- I know the DataObjectManager module includes a number of simple HTML editors, that could work if your HTML content doesn't need all the ins and outs of the complete TinyMCE editor.
- You could consider the use of ModelAdmin, although I'm not sure if including the TinyMCE field already works out of the box
But if you stick to using Pages, I really don't understand why you would want to keep them out of the CMS SiteTree, since there would be no way to ever edit/delete them at all...