Install Wireshark, nee Ethereal. Once it's installed, start a capture by selecting Capture->Interfaces. You should see your ethernet card named with its counters increasing. Click the "Prepare" button next to it. In the "Capture Filter" field, type "host empegbbs.com". Then click Start.

Press your browser's back button, press the forward button and confirm that you want to repost the data. Once the new denial page shows up, click "Stop". You'll see a bunch of data show up. Just click through them until you find the data that you intended to post. It may take some slight reediting, and it might be spread across multiple packets, but it'll all be there. You can right-click in the bottom panel and select Copy->All Information.

On other OSes, you can use similar methods with other network sniffers, like tcpdump and snoop.