Has there been any research on what buses in a computer can be analyzed while they are live, without rebooting the system to insert a proxy device? For example, I imagine I2C would be very easy to sniff because the wires are exposed, and because the data transfer rate is very low, but as far as I know, I2C transmits no sensitive information. On the other end of the extreme, I imagine PCIe and RAM would be almost impossible to sniff, even though the pins are all exposed, simply because of the high data rate (thankfully, I hope, because they both contain highly confidential information). The question is similar to this one here, but not specific to EM emissions.
My question is, what buses that carry sensitive data on a live system can be sniffed without interrupting the operation of the running system, if any? Sensitive information meaning anything that can be used to uncover the internal state of the system, such as cached inodes, encryption keys, ASLR layouts, process list, open sockets, etc. Pretty much anything in memory.