Hi Joel,
Plaintext does not see rtf files in the dropbox. And when I renamed an rtf file to a txt file, I could open the txt file but it started with all kinds of settings with '//'.
I am sorry, the name of the app is against you :-).
Another app, iA Writer, can export to rtf but does not see rtf files in the dropbox. Renaming an rtf file to a txt file gave the same effect as Plaintext.
However, Textilus also lacks search capabilities :-(. Smart Office 2, a $10 app that looks nice and handles a whole lot of different document types, has the same deficiency. What is it with these app developers: they never search for documents?
Maybe your best shot is using one app to search (like Dropbox or Documents by Readdle) and use the share function in those apps to open the document in your writing app of choice. It's an extra step, but I haven't found a better solution so far.
Since I don't edit rtf docs too much on my ipad (I stick with text files, easy and fast and do the layout later if I need to publish something) I have not looked any further than the apps I own at the moment.
I hope this helps!
Kind regards,
Dirk