Anecdotally, I've eaten with people before who have recited birkat hammazon to themselves quietly and who are either record-breaking speed speakers or who are using a shortened version. I suspect the latter, but cannot ask them (for reasons that are beyond the scope of this question) what it is that they said. I would just like to know how widespread is the custom of abbreviating the birkat hammazon and if it's permissible to do so under any circumstances.
This question asks for an online copy of the abbreviated text, and the PDF that Dov shared by way of an answer gives one particular version of it. I am assuming, from the author's prefacing his version by saying that he established it himself, that there is no standardised text in this regard.