I didn't work on any of the HD stuff. I mostly was responsible for non-tDM things while I was at digi, and Mix Plus hadn't even come out yet. But I do have some friends that were there. I have calls out to them to ask some questions.
However, from what I understand none of the 888/882 stuff will directly work with the HD cards. You can plug them into the legacy ports on a 96i/o, and 192 i/o. But you clearly have to have one of those available.
What I'm currently doing may be of interest. The 96 has light pipe in/out. On my nMP I have the Focusrite 18i8 which also has light pipe in. I am able to send 8 channels from the 96 into the 18i8. It works very nicely (well there's no word clock sync so you could expect jitter, but it's fine for a quick monitor action). Thus I believe you could use a 96 with the 8 built in channels, and use the light pipe for another 8 if you had something that would do 8 channels via light pipe--could even be an ADAT or the Tascam 8 tracks--you don't have to have a tape in them for them to be converters.

With regards to the card slots in the 192, and the 96. I believe there is a way to get 24 channels in or out with a full set. It can be analogue, or digital. This is the reason they went to a slot system. They could build one interface, and allow the customer to configure it to their own needs.
Tracks and channels are different animals--even with the digi stuff. For instance maybe you only have enough hardware to pull off 16 tracks, but you'd want more channels because of outboard gear.
There is an empty 192 on teh eBay right now. I don't know if it can function empty, but if it's like the 96 (and I expect that it is) you might be able to get 8 channels through the light pipe, and another two via spdif, or aes. I'll ask my buddy about it when I hear back from him.
