While looking at the MACsec information, I came across information about LLDP (Link Layer Discovery Protocol) and how it can be used for packet pacing. This is relevant for me as I found that when doing IPSEC encryption from one device to another, it is easy to overrun the receiver's packet buffers if the packets cannot be filtered and decrypted fast enough.
Mellanox has an article called Quality of Service, which discusses Data Center Bridging (DCB), Priority-based Flow Control (PFC), Pause frames, and shared buffers. They use the open source open-lldp. Their hardware seems to be capable of sending pause frames when buffers get too full, something which would help when slower SoC cpus are processing encrypted packets.
The open-lldp wiki has an article on Data Center Bridging (DCB) on Linux. It has very generic notes. But since there is a lldptool-pfc man page, it appears as though functionality is indeed available in Linux.