I'm in the UK so can I ask a question about HD? Why would you want to distribute them like this and secondly how many channels can a single card receive at the same time? I assume HD channels are mux'd like standard def or HD channels are here in Europe.
I'm not sure about what you mean by why would you want to distribute them like this, but I assume you are asking why the OP wants to have his media directors also be HD capture sources. In that case it is likely do to cabling. If they already have cables ran to various rooms in the house, then by having each media director be, in essence, a mythbackend, they can use multiple HD tuners without having to cram them all inside a single box; the upside is ease of having multiple HD sources, the major and obvious downside is that every machine must be configured, always on (when you expect them to be, or able to be remotely turned on at will).
One great alternative is the HDHomeRun which means no need to have every system in the house always on or to have multiple "backends" setup. Instead 1 backend with several HDHomeRuns or even a single HDHomeRun to do 2 HD captures at the same time..
Your second part about how many channels are muxed, well just 1 in reality. The stations here are a little new to the whole concept of multiplexing. Instead of showing multiple shows on the sub channels, they offer 2 versions of the same show: SDTV and HDTV which usually just translates to 4:3 or 16:9 and either 480i or 1080i/720p So, in general you need a tuner for every channel you want to watch as each station has it's own separate mux and show the same program on all of their mux's.
To say that visually:
Channel 2 is the analog version of 2.1
Channel 2 and 2.1 will show the same program, and if 2.2 exists it will show the same program in a lesser digital format (such as SDTV at 480i/4:3 ratio) while channel 2 will show the same thing but analog and 2.1 will show the HDTV 16:9 1080i with DD 5.1 surround of the same program.
Hope that helps.