Yup, it was something intended for 5.0, but we got too caught up in Joomla 3.0 support and the switch to the shared social plugins that we had to push the Pinterest button back.
We did some research before pushing it, and the biggest issue was simply that we need to generate the URL, description and image all at once to create the tag. It's honestly not that difficult, ut not something we wanted to tackle with all the other changes. With that said though, we're up for doing it now to get it ready for the next release.
Just to verify, for now, you're looking to have it added automatically to your articles with the JFBCContent plugin, right? If so, we can look into the changes required and help you implement it, if interested. It may take a few days (or 2 weeks or something), but we'd love to get something going to get some feedback before in a full release.
Thanks,
Alex