With the exception of the Entry Body and Extended Entry fields, all of the textareas in Movable Type are plain text inputs. BetterFields uses jQuery to convert all of these fields into WYSIWYG editors on the fly using the jWYSIWYG editor. The implementation is simple: call to jQuery and jWYSIWYG from the app and initialize it in the HTML header. All the transformations happen on the front end via Javascript.
Note: BetterFields modifies nearly all the textareas in Movable Type, so it does work with MTOS; but you're going to need Movable Type Professional if you want Customfields (which BetterFields also modifies).
Download the BetterFields Plugin (v.0.8) for Movable Type 4.x
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