A widget for the user's editor of choice

Some Linux friends were bemoaning all the apps that have their own
editors. Why can't you just tell the app which editor you prefer (as,
for example, one can specify vim in mutt)? There should be a simple,
direct way to do this in Tcl.

It seems Tk should allow us to put up a widget, spawn a CLI shell
inside it, and invoke an editor. I poked around on the wiki and there
are plenty of small editors, but not what I'm looking for.

TIA,

.