steveo, some thoughts:
1. Having high latency is not a unique problem to this interface; it happens in all sorts of applications. What if there was a way to determine the latency? Without being hooked to the vehicle, if I just plug in the USB to serial converter and then send a message and read the same message right back since TX and RX are connected together, can't we figure out the latency for a given hardware setup? Heck, just have the user turn the key off and wait for the bus to quiet, the software could figure out the latency even with the cable connected to the vehicle, if the PCM doesn't pull it to ground when off?
2. Then, watching the B-body idle traffic scan data, there WAS a repeating pattern of messages, and some of those gaps were significant. What if we listened to the bus and figured out the pattern, then How long a gap do we need to make a bus master message? How long is that message?
3. Knowing our latency and knowing the message pattern, couldn't we then insert a bus master request at the right time?