PRunderground has made a site change that now allows you to add a hyperlink in your About US boilerplate. This can be edited in your Newsroom under your account and gets appended to every press release you send.
So go ahead and add in a link now if you like. Please only add 1 link.
This screenshot shows it in the actual Newsroom.
And this shows the About Us boilerplate with a link in it appended to the bottom of a press release: