Hi Andy,
I have been using soft ports to implement a text widget in my GUI
framework. I also used GOOPS, which I regret to this day, and so the whole
framework needs a serious rewrite, but if you're collecting various species
to the museum of make-soft-port, you can have a look:

https://bitbucket.org/panicz/slayer/src/8b741c21b7372c24874d7cd1875e2aae3fc43abe/guile-modules/widgets/text-area.scm?at=default&fileviewer=file-view-default

Regards,
Panicz