In response to a request on this post here is a plugin I created to add TinyMCE to a textarea created in CustomPress:
I have pasted the whole code below. To make it work, simply paste in the field ID in place of my field ID "_ct_textarea_4fd2000002023". You can also add as many as you want to the code.
Be sure to check out the screenshots... and yes I am a bit proud right now.