Pownce has just launched a feature called "Pownce It! Buttons", which allows you to "create a button to send a Pownce note from your website." In particular, the "Link Button" version sounds like it will fit right in with the rest of the ShareThis social web service choices, or perhaps the "Send" options.
You can find code examples and structure definitions, along with pre-made button images, here: http://pownce.com/tools/buttons/
Thanks in advance for your consideration!