I know this was posted a long time ago, so you're probably all dead now!
I mailed HH today and got the same reply, so I mailed back explaining how good this is for the independendent learner and he got back to me to say he'll discuss this with the faculty of the Greek Institute this summer. It'll be too late for me, unfortunately, but all hope is not lost.
In the meanwhile go to this URL:http://leitourgeia.wordpress.com/greek-resources/
This chap has kindly prepared and revised a key for units 1 - 3 and number 4 is on the way, hopefully within about 3 weeks.
I would recommend that you let HH know if you want a key prepared. I sometimes wonder if he realises how good this book would be for self-teaching.
Hope this helps.