up to 120 8051 based smart transducers onto a single 4 core cable (Power, ground, "A" and "B" signals).

My question is how much delay should I put into the control signals to the LT1487's after the command codes have been sent to the 8051's (and vice-versa)?

I would start with making a '51 based master and connect it to the PC by RS232. While the B&B converters do, indeed, work very well, I would not rely on "activity controlled" turnaround in such a large system. That way you would have no "foreign" influence on your 485 bus. With a fully controlled 485 bus, you can turn in a couple of bit times. What I do is, wait 2 bits, turn the bus (two will turn) wait two bits and start.

It is crucial that all this is interrupt based, if you turn in the main loop, you will, most likely see that sometimes the transmitter start before the receiver has turnd.

Erik

PS:
you have read and applied 10 times to bulletproof RS485, I hope.