There is an editor named ReText for users of Markdown or reStructuredText which offers almost-real-time preview in a separate embedded window. However, ReText is not a very powerful editor lacking even simple search or search-and-replace, let alone more powerful editing capability. It would certainly be nice to have such a feature in Kephra.
ReText is written in Python as Kephra is written in Perl so speed ought not be an issue. The preview window is based on WebKit.
ReText is here: http://sourceforge.net/p/retext/home/ReText/