In addition to the "sub $1" cost of the micro itself, there's the cost of developing & maintinaing the firmware, programming, the micro, etc.

The trick here is to try not to do too much in the "satelite". A well designed limited capability satelite should have zero maintenance cost since no code change of the satelite should ever be needed to upgrade the product in any way. Had I not been using 3V3 processors, My satelite processors would have been LPC7 OTP. Writing the program for such a processor should take less than a day and - since it is totally "enclosed" becomes a true portable design. The keypad processor I mentioned in a previous post is an excellent example of an "OTP capable" satelite processor as well as being a truly portable unit I may use without ANY change in another product.

At the moment, I just want to know that I have covered all the options - at the moment they are:
The only thing I see you missing (and I do not even know that thay are not in the "bridge" chips category) is the fact that there are also parallel interface UARTs with up to 4 UARTs in the package.

re Roll your own
I doubt you can find enough FPAG/CPLD horsepower to make a remote UART for a price of less than double that of a small LPC that can do the very same - sorry Jez.

Erik