You should be able to use rendering tags for this by passing in a tag to the Html.DisplayFor() call, just as you can for a ContentArea. So, when rendering your XhtmlString property you can do something like this:

@Html.PropertyFor(x => x.CurrentPage.MainBody, new { Tag = "MyXhtmlStringRenderingTag" })

Then you can register the correct rendering method in your IViewTemplateModelRegistrator like this:

viewTemplateModelRegistrator.Add(typeof(MyBlockType), new TemplateModel
{
    Name = "MyBlockInAnXhtmlString",
    Inherit = true,
    AvailableWithoutTag = false,
    Tags = new[] { "MyXhtmlStringRenderingTag" },
    Path = "/path/to/my/block/partial.cshtml"
});