To help you, please elaborate on "special controller usage". Also, how far are the modules from each other. You might have a look at http://www.semiconductors.com/i2c/suppo.../#appnotes and see if there is anything you can use. I know you are asking for the below, but have a look anyhow. One often missed fact about IIC is that when all nodes are uCs you can run at any speed you can make the processors respond to.
Must be like "PCI or ISA bus on the PC" If you do not need more RAM than e.g. the 89C51Rx2s provide (1k+) you will have P0 and P2 free for bus signals and the task of a parrallel bus becomes relatively simple (at least up to about 10 modules if it is a backplane).
Again, to go beyond the suggestions above let me know what resources you have free for bus use (port pins etc)

Have fun,

Erik