PT has a cross platform copy/paste plugin for CS and being open source I'd think it would be possible to use. Not a gui mod as caarrie said but definitely doable in other ways.
PT uses CEGUI so I'm not too sure they wrote the copy and paste code themselves. Then again I'm looking through CEGUI's source code and I see the following in its editable text boxes:
/*************************************************************************
TODO:
Clipboard support
Undo support
*************************************************************************/
EDIT: OK so PT did write their own clipboard code, and they did it as a CS plugin so it should be reusable by
any CS project GPL-compatible CS projects. What it does is allow you to store and retrieve textual data to and from the system clipboard regardless of platform since they implemented it four times. But you're still stuck with implementing text selection within the text box, and it seems CEGUI allows this but PAWS does not. Also, there is no undo support.
EDIT 2: I just realized their plugin is licensed under the GPL so it's not like they're gonna hand it over to Crystal Space or anything (which is LGPL) unless they relicense it. So in reality it can only be used by GPL-compatible projects and not necessarily projects that use Crystal Space. That still means PS can use it, but that makes my previous edit incorrect. Also I'd rather it be a part of CS, and not have to download PT code to compile PS.