In the ideal case, the only tri-state bus is the external data bus.

Inside of your design, you have two data buses. These are an input data bus that is only used for writing to registers, and an output data bus that is used to read registers. Since there is only one input source (the data pins), there is no need to tri-state the input bus. The pins always drive it.

assign ctr_din = data_io;

The output bus really isn't a bus. You use a mux to select which register you want to read on the external data pins. A final mux determines if the device output buffers are to be turned on.

assign internal_bus = read_a ? data_a :
                      read_b ? data_b :
                      read_c ? data_c : 
                               data_d ;
assign data_io = (!cs & !rd) ? internal_bus : 8'bz; 

Note that all internal nodes are defined, and the only z state is on the data_io.

I use the split bus technique to implement the SFR bus in our 8051s. (This ties this thread back to the 8051 topic.)