Teach Varnish to write letters

We've had people typing response letters for quite a while, but it's slow and costly, and the only difference in the letters is the greeting by name. We also can't change it to an impersonal response like "Dear Customer".

So how can we make this more efficient? Let's make lots of copies of the letter without the name, leaving a hole in it's place. Then when we get a request we'll use a special process