(1)
You should be able to use a 8051 microprocessor for this. The key question is does your microprocessor have to be compatible with an existing communications protocol on the RS485 line, or do you have the freedom to write the protocol anyway you want?

Some of the 8051 upgrades have an enhanced UART which allow you to do automatic address recognition. If you plan to have a RS485 multipoint LAN, this would save you a lot of processor time.

This option allows you to setup the UART with a recognition address. The microprocessor can run its program without constantly polling the data on the RS485 LAN, because the SFR register setup can issue an interrupt when the protocol addresses that microprocessor.

(2) Multi-Master:
There are various ways to design this. You might consider the simplicity of a single master that polls the slave micros for xfer requests.

If you multi-master, you might have Rx interrupts off until you're in active communications, when you're inactive and want to test the status of the RS485 LAN, you could treat the Rx Byte ready flag as a debouced switch, polling and resetting it to see if a period of inactivity exists before grabbing the bus. Its best to use a random delay so all micros don't wait the same delay and then attempt to talk at once.

You might try a token ring approach wherein a fotware hand-off is constantly going on. Each site awaits reciept of its change to talk in ordered sequence. If a site has nothing to say, the inform the next site that it can talk now if it has anything to say. If a site is ready to intiate a master transaction, it waits until its turn and then does its business and then passes the control to the next site. You can visualize the way it works as thinking of a crowd of people with one microphone to communicate. People say what they want and then pass on the microphone to the next person, who may or may not say anything before passing it on.

Hope that gives you some good ideas,
-Jay C. Box