I have considered using the start bit to give the timing of the link
(baud rate) which as you say will only work for 50% of characters.

I was also thinking in terms of timing edge transitions and dividing
this time by the bit time to give number of bits at same level (mark or
space). Has anyone else considered this rather than the more conventional
measure the link at bit rate aftre delay of bit rate/2 from start  ??

I hope some of this makes sense !!!

Rasher