The PlainTextEditFindBar is already exported, just do the same for RichTextEditFindBar.
I am currently porting KJots to KPIMTextEdit, as it seems to be much nicer compared to KTextWidgets. One of the features I like is inline search, which I would like to have both for KJotsEdit (which can be RichTextComposer) and KJotsBrowser (which is QTextBrowser). Since QTextBrowser is a subclass of QTextEdit, having RichTextEditFindBar there would be useful :)