I think I have worked out how to add an RTS line to the UART based serial if your interested.
My links are full duplex with two UARTs so I don't need to turn the line around but this is often a gotcha for people who think that the last byte has gone when it's really still in the chamber.
There is an interrupt available that fires when the last bit transmitted is done,
The "USART, Tx Complete" interrupt. I think there could be a mod to the hardware serial code, maybe you should create an RS485 library.
That said there are 485 shields available
Do they handle the TE signal as you have mentioned though? I've never looked at them.
What I'm doing is a custom board with the RS485 transceivers built in so I don't need a shield.
they're 8 legged SMD
SOIC8 IIRC, pretty big by SMD standards 
I think protel was the program I used to use.
I cut my teeth on the original DOS-based Protel, we used to get pre-release versions from the guys in Tassie.
Most people use Eagle these days, the UI is total crap but at least there's plenty of people around to help. I don't know if it works on Linux or OSX though.
Rob