Hi Terry, yep things are progressing slowly. I keep being distracted by other stuff but I now have a partner in crime (a bloke in NY state) who is backing the hardware manufacturing so that's keeping me focussed.
The first "product" is a Leonardo-compatible board with the RS-485 networking built in (LPC co-processor) and provision for IO modules to be plugged in. After that we have ideas for about 20 IO modules to get the ball rolling.
The Arduino dongle interface is still current but on the back burner for the moment, it will have to be done before long as well I suppose to allow standard Arduinos to connect to the network.
What I could use right now is someone else to look at the schematics and find the bugs or suggest better ways to do things, there is also a rudimentary document describing the system that you may be interested in. I would certainly like to have 1-2 more people involved if possible.
If you want to look at the docs let me know (PM/email/here) and I'll send them.
@Al
I am rather pleased that you didn't say I was just talking rubbish
No, I think you are on the right track and in fact what you propose is very similar to one of my earlier (never implemented) designs.
I am essentially talking squirt and prey with everything being a global broadcast, if I need ACC functionality of some specific chunk of important data then I will make that a logical function of the control system as opposed to a function of the protocol.
I think that's reasonable.
I had envisaged a network power source that pulled both differential lines high which means that the devices will only ever pull low,
Yep, if you apply normal "fail safe" biasing to an RS-485 line you will get it to idle high with no enabled drivers. What you then have to do is wire the transmitters in a slightly unconventional manner. An inverted version of your data goes to TE and the transmitter input is hardwired to GND. That way when you are transmitting a 1 the transmitter is disabled and the biasing pulls the line high, when you transmit a 0 the transmitter is enabled and it transmits the hardwired low.
You cannot get full speed like this but it doesn't sound like you need that.
I would be interested in your thoughts RE implementing CAN-BUS as is or designing a simplified version from scratch.
To be honest it's probably better to just use CAN chips, but CAN has a limited range (40m/130' IIRC) and anyway I'm a DIY kind of guy 
As you can probably tell I prefer to design stuff myself, not the most efficient way I suppose but you learn a lot more. So as long as this isn't for a commercial product with a tight deadline I'd go for it.
Rob