You can have many different terms that react to LazyApproval: but as far as I know, only two outcomes that these terms will result in, which are the two you are seeing.
But if you have custom outcomes to a task that you added to a Flexi task per say, then if you respond to the email with Pending, then it will accept the answer as pending and correctly complete the workflow with Pending as the outcome. With custom outcomes you just have to match the word exactly. In the lazy approval settings for the farm, you would not need to create a synonym.
This is gold Andrew!
Just to add to this gem - the response does not have to be case sensitive. i.e. the responder can match the world exactly but does not have to worry about case.