Append trailing .html on posts (on the fly)

I’m developing a plugin and I’m looking for a way to accomplish what the plugin “.html on pages” does, but for posts instead of pages.

“.html on pages” source: http://plugins.svn.ww.wp.xz.cn/html-on-pages/html-on-pages.php
Plugin page: http://ww.wp.xz.cn/extend/plugins/html-on-pages/

The thing is that I really want to avoid changing the permalink structure, since it will just be in a specific theme that I need this permalink structure (a mobile theme that it switches to on-the-fly).

Anyone got any clues for me how to accomplish this?

Thanks in advance!