I maintain custom multiheaded mythtv frontends for various friends as well as one here at my home.
For SD standard usage You can easily service 4-5 LCD's from a modern system. My home system runs 1 HD frontend on a projector and 2 SD frontends on LCD screens. All equipment (front and backends cable/satelite boxes etc) are stored in a custom media cabinet Using IR extenders and VGA Multiswitches you can easily switch programming to all tvs or certain tv's. This is also very usefull for DVD Jukbox systems. You can easily play 4 or more DVD's on most modern systems and with nvidia binaries you get XVMC acceleration on all displays including TVouts with there multiheaded cards. This way you get access to your 100;s of dvd's to all rooms with 2 PCS, 1 backend, and multi-frontend.
Most of my friends bulding new houses are requiring no equipment except LCD and speakers in their rooms for a clean environment. The VESA mount cases from fire are a nice solution, but i think for many rooms the saving could add up.
I evision being able to pull this off with MCE and realise it will be alot of work. It allready gets confused on multihead setups. One my dualhead Nvidia card it will only use the TV out if i have a monitor pluged up.
If anyone has interest in this or any ideas i would appreciate it.