Bus value if some pins are floating
I'm sorry, but you're out of luck. There is no obvious way to tell if a digital input is floating. The bus receivers cannot tell if a given input voltage level is occurring because a driver wanted to, or if it's because there is no input driver. All it knows is that the input is either higher (logic 1) or lower (logic 0) than the receiver's threshold voltage.

It's certainly possible to build such a circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

In this case, Vth is the nominal logic threshold voltage, and R4 is a largish resistance. R1/R2/R3 set the upper and lower error bounds for the input voltage.

In operation, a valid high or low on the input will pull it either above or below the comparator set points, and both comparators will read either high or low. This sets the XOR output low, indicating that the output is good. If the input is left floating, R4 pulls the input to an intermediate value, and one comparator goes high while the other goes low, and the XOR output is driven high, indicating a floating input. 

Needless to say, I doubt very much that your digital inputs look like this.