Sure, you can roll your own provided the shield doesn't interfere. I'll have to order one and see how accessable the pins are for the RS485 connector. Plug-n-play would be even better but I suspect that won't happen unless I design my own ethernet shield.
Graynomad - I am primarily designing for security but the control portion will likely be coarse granularity. I haven't completely thought that all out yet. I have some low voltage (12vdc) controls already in place (light switches, etc) so the interfacing should be fairly simple. I may have to mux a few things if my granularity gets too coarse.
I'll share more as I make design progress.