For the communication path MTBF, you would need to know which device failures could affect it, and their likelihood. You also have to decide whether continued communication with a host system is necessary, or whether the system can continue to operate without it. Common-point failures such as a non-redundant power supply might be the major item. With redundant power supplies, the chance of physical damage to the bus might be more important.
MTTR should be estimated on the basis of replacement of failed devices, rather than repair. If a spare device is available, this can be done within a few seconds (as far as the bus connection and communication functions are concerned; physical installation is likely to be the limiting factor).