When it comes to sales I'm a firm believer in picking up the phone.
That's not to say 7878's method isn't a good one. Far from it - it's a very good one. It's creative, it's ingenious and it's memorable - in fact I bet more dentists now have a set of red dice than any other profession in the US (not from 7878, but from the copiers who couldn't even be bothered targeting a different profession).
I'm not in ORM, but if I were, I would still definitely call clients directly.
I'd do my research, build a list of target clients, work out what my 'hook' is going to be and then get on the phone.
The beauty of speaking to clients directly is you learn something from every call you make (both the good ones and the bad) and can make immediate changes to how you structure your calls, so every call is a little better than the last - you can't do that as quickly, or as cheaply with mail outs.
And a hook for ORM calls would be easy. I guarantee if you ask the client what they thought of the (shitty review/bad press/scathing article) that just appeared online, you will have their undivided attention.
And you'll have more success selling your ORM service to them at the exact moment they are shitting their pants, than on a follow-up call 3 days after your letter arrived.