preserve custom tags through GetXHTML() ?

I use custom tags in my HTML code for my LFO extension 'selective content' (more about that on http://fckplugins.saulmade.nl/ > 'LFO plugin' pane > 'selective content - loading specific parts'), but now when I load the whole content of a file with custom tags in it and try to save that content, the custom tags with their content are lost through GetXHTML(). Is their a way to define tags that must remain untouched?