Hello @kfriesth, in order to implement these kinds of custom transports I guess that some minor changes will be needed at some points. Are you planning to use the Arduino IDE for programming this Teensy 4.1 or you have another kind of custom build system?
I think that using Ethernet or EtherCAT in Teensy 4.1 would be interesting despite the need of having to purchase an external RJ45 connector.
Let us know if you figure out how to embed your transport library. Here at micro-ROS team, we will be happy to help you to integrate it in the micro-ROS stack.