Multi-master and multi-drop == collision detection which can be a problem, but if you have low speeds and infrequent transmissions it won't be that hard.
FWIW myself and another bloke are working on a sensor network that will I think do exactly what you want. It's not multi-drop (redundant ring) but is multi-master. It won't be ready next week 
but if you're interested email me. The scheme we are working on should allow a combination or wired/wireless as well, the idea being that it is essentially a wired network but you can have sub-networks connected wirelessly if there's a difficult place to get wires to.
It may be good to have 1-2 more people on the "team", anyone interested please email me as well.
Rob