Re: Selecting sub-boards
yes, you can put as many boards as you want. Just two pins dedicated to the enumeration (one input and one output). Also you'll need special protocol to form packets with addresses.
When you make a short pulse on the first enumaration pin the chip will go into ENUM mode, the packet it gets immediately after this will be considered as the address of the unit and will be stored into EEPROM. The just enumerated device will response with some acknowledge code to show the master everything is OK. A pulse on enum line will be generated so the next chip will go into enumeration mode.
You can also have some protection against accidental enumeration - the enumeration could work only 5-10s after power up