The good folks at Harmony customer service sent me the following answer a couple of days ago. I've been a bit slow copying it here.
Dean
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Hi Dean,
Thanks for your note. RemoteCentral.com does have some very well-informed customers answering posts but it is a good idea to email us as well. Regarding your first question, it isn't possible for users to create new device types. That's a process that we take care of here and it is certainly a device type that we'll be looking at adding to our database sometime in the future. For now, you could add the device as a Cd player, learn the extra commands to the Harmony and then map them to the various buttons on the SST-768.
From your member home page, click on 'Add a Device', select 'Cd Player' and follow the prompts to add the CDR 880. As you've noted, it won't find it in the database so you have the option of choosing a similar device. It will prompt you to give it an infrared command to see if it matches up with one of our known infrared languages. After that process is done and the CDR 880 appears in your device list, you can click on 'More Options' and then 'Learn Infrared Commands' to teach the database the functions that it doesn't already know. The IR codes are then uploaded onto the web server so that subsequent customers will have another device to choose from. While in the past teaching a new device was time consuming, new software now allows our customers to enter new IR codes with fast digitization. It now only takes minutes.
With regards to your second question, you shouldn't need to edit any XML to setup a turntable. What you can do in this situation is set the turntable up as a CD player (Sound familiar?) and during the prompts choose the manual options (ie: you don't have a remote that controls it). This is a bit of a roundabout way to do it but it will work. I see you've already created a 'Listen to Records' activity so once you've set up the turntable as a device, you can go back in an edit the activity by clicking on 'More Options'.
As for the Mac question, it is something that we're working on and we are committed to providing support for Macs in the future, but unfortunately I can't give you any definitive ETA.
I hope this helps. Please don't hesitate to contact us if you have any further questions.
Regards,
Harmony Support Team
www.harmonyremote.comThe Internet Powered Universal Remote